Not everyone has time to actually play through an entire game or work through it. Wouldn't video games sell more if they provide an option for the casual gamer - aka unlock everything, so someone doesn't have to work their way through it?
Nowadays, I just resort to watching gameplay walkthroughs on YouTube because it allows me to multitask and still immerse myself and see the story.
If I was given god mode, I'd probably actually buy the game.
Nowadays it's more conventional to treat these assists as accessibility settings, clearly listed in the game's menus, advertised in its promotional materials/website, and covered in accessibility reviews, rather than secret codes that need to be discovered.
That way, players who are interested in the game but don't have the time, physical, or mental capacity to play the entire game on its default settings can more easily learn if there's a way to play it that will work for them, before they buy, and without needing to get secret insider knowledge from other players.
Game studios do this not just out of the goodness of their hearts or because it's the right thing to do to make technology accessible (though those should be reasons enough). It also increases the size of the market they can sell to, because as you point out, if a player doesn't have confidence they'll be able to play and enjoy the game, they don't buy it.
Modes similar to god mode go by a variety of names including:
But the common element is that you either cannot die, or it's at least vastly less common for that to happen (eg. way more health/damage reduction, or super-fast regen), and the penalties for death are reduced to practically nothing.
These modes or related settings may also power up your abilities so you can more easily defeat enemies, overcome obstacles, or have less need to manage limited resources like ammo/mana/money/super meters.
Some games go even further and include some amount of auto-playing feature so you can "give your sibling the controller" digitally and watch it play past a part that's giving you trouble or that you're just not enjoying. Nintendo in particular has been exploring this, starting with the Super Guide in New Super Mario Bros Wii and continuing into, Mario Kart 8 Deluxe, etc. This has the added improvement over watching YouTube walkthroughs where at any point you can take back control to play the parts that work and are enjoyable for you.
So, the tradition of god mode and infinite ammo codes didn't completely go away, it's just changed form into one intended to be more discoverable, to serve exactly the kind of player need you've identified.
God modes were originally more of a developer mode, which would, by some slip of the tongue or through enterprising gameplay, escape to the end player. (Think of the Konami code for a famous example.) They come with ups and downs, but the tradition still somewhat exists.
The biggest drawback is with multiplayer games, particularly (but not exclusively) in cases of Player vs. Player; great investments have gone into preventing God codes / cheat-codes in these, not because they might make it more fun (or sometimes less fun) for one player but because they can ruin the experience for someone else.
That said, offline games typically have a flag which will enable a developer console, if you need one; and this, in turn, allows for spawning in ammunition, items, or NPCs, toggling invulnerability, or toggling clipping. It's usually locked behind a launch option, which is typically fairly easy to find on the web.
As an example, for Doom 2016 it is readily available with Ctrl+Alt+~, but only in single player. If you play multiplayer, the console won't work. After that, there are lists of commands you can type in to get what you want; iddqd being the command for god mode.
The other common option is to have a separate program running with shared memory, which can enable cheats independently from the game. This is common with games like Metro Exodus. In this instance, it's usually possible to find such a tool available on the web, but I strong advise against running anything you aren't absolutely sure of the authenticity of. Again, it serves the purpose of making the game more readily testable, and easing things for players who just don't have the time to get really good at it. However, anti-cheat programs will identify such programs very quickly when online.
The last thing to consider is the onset of the age of modding, in which many developers have gotten increasingly open to user-generated content for their games. This often includes mods which make the game significantly easier, or even game-breakers; but if there's no online competition involved, that's usually a safe assumption.
Hopefully this will make modern games a bit more navigable for you!
I would like to add a slightly different perspective to this.
When publishing games on console, all game content, modes and options have to be listed as part of the submission and are subjected to (what these days seems to be theoretical) testing by the console manufacturers prior to release.
If a developer has added some semi-hidden cheat option, the functionality that is offered still has to be tested. If a bug is found that only happens when the cheat is running, this can delay the launch of the game. Why risk delaying launch by leaving code that was really only written to aid development?
Most code is developed under environments where there are "build configurations" that allow certain routines only to be included and executed under development environments (for example writing text into a log file to help understand what might have happened when a rare bug occurs) but this code is not present in final submission builds. It makes sense to include "cheat modes" in this code for the reason stated above.
For reference, I included an "autoplay" cheat in a game I submitted to Sony which demonstrated that all the Trophies could be collected (some were pretty hard to get) thinking it would speed up submission as it would test the Trophy collection for them. They rejected the submission as the same cheat then violated one of their rules that Trophies must require effort from the player to earn. The cheat was intended to ease the submission process, but in reality it delayed it.
In the early days of game development, few knew how to make a balanced game, and of those that did, didn't always have time to meet their launch window and make it balanced; most games were in the category of Nintendo Hard (warning: TV Tropes). Programming was also time-consuming, so it was often easier to program a cheat code in for testing, and it didn't always get taken back out again. The same is often true for other kinds of assets as well; many times assets like sprites, animations, music, and code are left-over on the cartridge, disc, or inside the game files. These are left-over artifacts from not having enough time to do everything a developer wanted to do.
The first major blows to cheat codes were the introduction of difficulty options and improved developer tools. Developer tools allowed for faster build times, more advanced programming languages, etc. Cheat codes were less necessary, because developers could quickly change memory and code. Difficulty options basically split gamers into two groups. The hardcore gamers that thought the challenges were rewarding and there shouldn't be an easy mode, and the casual gamers that thought a game should be accessible to (almost) everyone.
If a hardcore gamer-programmer had to introduce an easy mode for marketing reasons, it was usually in a mocking manner, like Doom Guy's baby pacifier face on "I'm Too Young To Die" difficulty. Much later, more accessible games would introduce a casual player's quick mode, called "Story Mode", with limited or no combat and no challenge whatsoever. This is the mode you're likely thinking off when you talk about a game you want to just enjoy quickly. At any rate, almost any game with difficulty modes was already easy and fast enough for a casual player, so introducing cheat codes wasn't necessary. If a game had codes, they were easter eggs or cosmetic differences.
Eventually, DLC became readily available. At first, it was just cosmetics or maybe a decent starting boost to the game. After a while, games were just designed to require absurd amounts of grinding that was fixed by paying some price for a triple-experience DLC. These games were designed to frustrate players, so no cheat codes here. They then moved on from this to energy meters and progress meters that ran in realtime, which you could advance faster by purchasing in-game premium currency, known as microtransactions. They also started adding loot boxes and battle passes for extra rewards that would move you along faster, and other pay-to-win mechanics. Cheat codes are antithetical to the concept of pay-to-win.
Also, keep in mind that many modern games are also live services. That means that you can play with other players, and you'll see their cool items and abilities, and want them for yourself, so they'll offer loot boxes and battle passes and everything else to entice you to spin money so you can be powerful and/or cool like that one person you saw.
In addition, a lot of game engines had "mod" capability. Anyone with skill could make a game do pretty much whatever they wanted to. You no longer had to cheat, just install a file to customize the game the way you want to. Any game with this system was also unlikely to have any built-in cheats, since the players could already customize the game however they wanted to. It simply didn't make sense to include a cheat mode. Also, games started introducing multiple game modes, like Minecraft's Creative Mode, where a player could build without fear of dying.
Most modern games don't have cheat codes (except maybe cosmetics) because (a) they would not be used by the target audience (e.g. hardcore gamers), (b) the game already has a ton of accessibility features and difficulty levels, (c) the game is intentionally grindy to extract dollars from your wallet, or (d) the game is nearly infinitely customizable anyways, so players can completely customize their experience.
As far as your specific idea goes, there are few cheat codes in modern games out there because there isn't any incentive to have them in a game in the first place. Hardcore games are not designed for casual players. Games with run long times don't really have a way to shorten the story they want to tell without cutting out critical plot elements, though many will let you skip some or most cutscenes, but that tends to leave first-time players confused. Casual games typically already have more than enough accessibility features without needing hidden codes. Microtransaction games intentionally stretch out the game time in hope of earning more dollars.
Also, cheat codes used to be hard to find, but today, you can find virtually any cheat code in seconds on Google, so that takes a lot of the fun out of it for many people. There is a small niche market with people like me that like discovering secrets and bugs, but even I would typically rather just have a menu option than have to find the code somewhere on the Internet just to play a game.
Actually playing through the entire game is the intended experience* (even if that wouldn't be the best experience for some players).
Developers don't cater to things outside the intended experience if they feel:
It would take too much development time
Providing something like god mode should not be that difficult, but even for something like that, one would still need to test how that interacts with the various mechanics the game has and test that everything still works (if there are, say, bottomless pits, and falling into one would trigger death to cause a respawn, removing the ability to die might mean you just keep falling, so you may need to specifically handle that, or the game may trigger death for physics interactions that would otherwise cause the game to break, which developers may or may not know about).
Other functionality may require a whole lot more development time.
It provides too little benefit
This benefit can be monetary, for the sake of accessibility or inclusion, or anything else.
Also note that working through the various challenges in the game may make up the majority of the typical play time. So when a game is developed and priced with that play time in mind, developers may not expect many to be willing to pay that same price for a much shorter experience that only goes through the story, which would decrease the perceived benefit of such features.
It is contrary to their artistic vision
One game (series) that's infamous for being very difficult and having no difficulty setting is Dark Souls, where the intention is to "bring everyone to the same level of discussion and the same level of enjoyment" (which seems like flawed reasoning, as different players have different skill levels and physical capabilities, if nothing else, but I digress). Their artistic vision was clearly for players to have a very specific experience, and they didn't want to betray that.
* Side note: in a few games, during difficulty selection, one of the options will say something like "this is the intended experience".
I'm not saying I necessarily approve of these reasons, and there are plenty of games that provide plenty of possible solutions that allow you to enjoy the game in this way (as mentioned in other answers). I'm merely explaining why developers may choose to not provide any such solution.
Most developers approve of content creators playing their game for the enjoyment of others, so if this lets you sufficiently enjoy the game, I don't see a problem there. Supporting such content creators (possibly just by watching them) may also indirectly support the developers, as this increases the exposure of the creator and may influence them to keep playing that game or sequels, which can serve as marketing for the game or developer.
As the other answers have talked about, cheat codes have morphed into open ways to customize the game, make it more convenient, and adapt to wider play styles. A more few examples which are so ubiquitous we don't think of them as cheat codes anymore include...
Plenty of games have an easy mode where tricky actions become simpler and the amount of grind needed is reduced.
Some games have PVP where a god mode would be a deal breaker.
Some games have microtransactions where a god mode would be financially harmful.
In the pre-Internet age, cheatcodes had an important advertising function. Back then, the main source of information for gamers on what games to buy were print magazines. One standard section of these magazines were the cheatcode sections where they listed all the cheatcodes for games released in the past couple month. Stumbling over the cheatcodes for a game in the cheat section put the game back into the mind of the reader. Which meant that being mentioned there was free advertisement!
So it was common practice back then to wait for the magazines to review your game, and then send them a letter with the cheatcodes so they could print them in the next issue.
But then around the year 2000, every gamer got Internet access. Print magazines were replaced by gaming websites. People do not read gaming websites front-to-back like they do with magazines. They only read what seems interesting to them. Putting cheatcodes onto a website doesn't help to reach an audience of people who don't already own the game. And if people wanted cheatcodes, they went to specialized databases like GameFAQs where they specifically looked for the titles they owned.
So it's no coincidence that the beginning of the internet age is also the age where cheatcodes started disappeared.
I would also add to this that a lack of cheats and god mode makes completing the entire game take longer, having to redo harder sections the player fails at, over and over, therefore adding to the perceived 'value' of the game.
Plus the 'damn I am good' FEELING when a player fails many times to complete a section and then FINALLY succeeds.
I know the right answer to this: this first game code appears because the developer couldnt pass the game level to check the next one... so based on this idea, games arenot that dificult anymore, so gamecodes no longer need bcz ppl can pass any game, it is just a matter of time
I believe because 20 years ago we all were noobs on playing games, and we needed cheat codes then, nowdays most of the gamers already have large experience on gaming what makes many games be easy even tough those are harder then games 20 years ago. Bassically we learn how to play and those are no longer hard for us to make cheat codes. I remember I was using an program called ArtMoney for getting ingame money >)
