I see it all the time, in RPGs, FPSs, and more. I never see it talked about though. If level represents prestige, why is a linear progression not used? You can't easily determine somebody's experience that way. I imagine it has to do with something psychological, but I'm unsure. Where does this standard originate from? It's a bit of an open-ended question, so I'm open to responses. Thanks.
TomTsagk's answer already covers several major reasons. I just want to add on a few other ways of thinking about this:
Feedback loops: Counterbalancing power and skill growth
Typically in an RPG, as you level up, your character gets more powerful, either through stat growth, gaining new abilities, or collecting better gear. This makes them able to deal more damage or take more punishment before you have to retreat and heal.
Alongside this, the player themselves tends to get better at the game: learning enemies' weaknesses, mastering timings, finding optimal strategies. So their efficiency in overcoming similar challenges goes up.
Left alone, this will tend to produce a runaway positive feedback loop: you gain a level, which gives you more power and skill, which helps you reach the next level faster, so you gain even more power and skill, so you reach the next level even faster than that...
You'd end up with an accelerating rhythm of progression, where the first level is the longest and slowest, and they come faster and faster after that, until in short order you've burned through all the meaningful progression milestones the game has to offer. This is generally the opposite of what you want. You want early milestones of progress (and especially the first one) to come quickly, to encourage a player who's just started dabbling in the game and teach them about the systems. Once they get deeper into the game, they're more invested, and have the patience to wait a bit longer between levels, so they don't exhaust all your game's goodies too quickly and run out of challenge or future milestones to look forward to.
Increasing the XP targets is a negative feedback loop that counters this runaway acceleration. The more you level up, the harder it becomes to level up. If calibrated well, the curve resulting from these two interacting loops closely tracks the player's investment in the game, starting with rapid, exhilarating progress, and then gradually spacing them out as the player gets into the groove and doesn't need the hand-holding/cheerleading, and can self-direct toward goals on a longer time horizon.
Adjusting the XP targets at each level gives designers a lot of flexibility to fine-tune this curve as needed for the game's specific experience goals.
Why not reduce XP earning instead?
Mathematically, increasing the XP target you need to hit, or lowering the XP you get from a given enemy/task as you level up, both achieve the same end: you have to do more of that task to make the same progress, or you need to seek out more challenging and rewarding challenges. So you could do this, and it would work.
The reasons to prefer increasing the targets are mostly psychological and aesthetic.
Loss aversion & negativity attention: humans have a tendency to fixate on the negative, losses, the rewards they're not getting. This can make decaying rewards very demotivating — "yesterday this enemy gave me 100 XP, now it gives only 90 XP? The game stole 10 XP from me! It's not even worth my effort anymore."
World of Warcraft's experience with Rest XP is the classic example here. They started by reducing XP rewards when characters ran out of rest, but that was deeply unpopular. So instead they gave bonus XP for well-rested characters, and increased the XP targets to compensate. Mathematically the situation is identical, but psychologically this is much friendlier, and was better received by the player community.
Integer limitations: players in RPGs tend to expect XP to come in integer amounts. This limits the granularity with which you can diminish payouts. An enemy worth 10 XP at base gives you only 9 steps to work with before it becomes a worthless 0 XP. If the ideal balancing math says that at this level is should be worth an amount of progress that maps to 6.667 XP, you have to round one way or another and deviate from your ideal curve. You can counter this by making your XP numbers big, but it does look a bit weird seeing huge numbers like 600 XP in level 1, which ties into...
Numbers getting bigger is motivating: as TomTsagk's answer mentions, it feels good seeing numbers go up as you progress through the game (and at this point, there's a lot of gaming history contributing to player expectations that they should go up). If at the start of the game you were defeating foes worth 10 XP, and now you're defeating foes worth 1000 XP, wow, you're 100 times more powerful than you were back then!
Without this, players can experience a "red queen effect", where it feels like, for all their effort, they're not really progressing — they keep fighting harder and harder enemies for the same 50 XP that the rats in the tutorial level gave.
Less burden to explain: this is a minor one, but if you start shrinking players' XP earning, they tend to want to know why. What rule says how much XP they'll get from doing X at level Y? If you have a very hand-tuned progression curve you might be hard pressed to explain this to players' satisfaction, concisely. But an XP target, by convention, is easier to accept as an opaque fact: it takes longer to level up because you need 10 000 XP to reach level 5, because that's what the number on the XP bar says.
Bonus: keeping players together in multiplayer
For multiplayer online games, the negative feedback loop provided by increasing level targets also acts as a bit of a leveler between players who've been playing the game a long time and those who joined more recently.
Say a new game comes out and I start playing it day one. For two weeks I'm raving to you about how awesome it is and begging you to make an account and play with me. Finally you join up, but now you're two weeks' worth of play behind my character. Say the gap between starting level 1 and my character at level 20 is 200 000 XP. That's huge! We can't play any dungeons or bosses together with a 19-level gap between us.
But as we both keep playing, earning XP at similar rates, that 200 000 XP gap starts to count for less and less. By the time I hit level 30, each level might be 100 000 XP apart, and so in a couple weeks you've caught up to within a level or two of me — without me needing to stop playing to wait for you.
At scale, this effect makes it easier for cohorts of players to stick together and find people to play with within relevant level ranges, despite the diversity in players' play schedules and histories, and the attrition of high-level players lapsing from the game. It's not a complete solution on its own, but it's one more tool in the designer's toolkit to mitigate these problems. This is why you'll often find online RPGs have particularly extreme ramps in XP targets from level to level, compared to their single-player counterparts.
There's a few reasons why this happens:
Make the experience less predictable
If a user goes from level
1 to level
10, with a linear scaling, they will have a good grasp of how the next
90 levels will look like. This makes the game boring and predictable.
When leveling is non-linear, it makes things harder to predict, allowing the user to enjoy the experience, and kill as many enemies as it takes to level up.
As mentioned in the comments as well, how would this be balanced? If a user can kill
100 weak enemies to level up, what motive do they have to go for stronger enemies? This means that the fastest strategy to level up is to keep killing the weakest enemy, so any user who decides to fight stronger enemies is punished by slower progress.
Increase the scale
Your XP requirements goes up, your stats go up, your health goes up, everything is getting upgraded. From being level
50 HP needing
10 XP to level up, you are now level
1300 HP and need
5000 XP to level up.
This makes the game exciting, the user feels like they are on a new level. Old enemies are now easy to kill, and new enemies are a challenge.
Having said all that, there is no one correct answer on how to handle leveling. If you are really motivated, you can make a leveling system that is linear and as long as you know what you are doing, you can make it work.
Needless to say it's up to you what you choose for your game.
In skill-based games like League of Legends, non-linear leveling plays a role similar to "progressive overload" in weight training. That is, instead of continually doing the same thing to progress (lifting the same weights for the same number of reps/sets, or killing the same enemies), the player has to take on more difficult or complex challenges to keep making progress (heavier weights/more reps/sets, or killing tougher enemies). Stagnating at the same challenge level will lead to diminishing returns and slower progress.
Non-linear leveling is also used to slow down progression and keep players playing longer before maxing-out the endgame - especially in MMO games (where players might be monetized through subscriptions or microtransactions the longer they play).
Myself, I prefer games with horizontal leveling or no leveling system at all, to encourage player freedom, as seen in Minecraft, for example.