There are several reasons why computer opponents need to be dumbed down.
Philipp has mentioned the part of keeping it fun already, but another worth mentioning is balancing for computer opponents‘ unfair advantages.
We are not able to make a true AI that plays like a player. The computer opponents we face today (even when called AI) are not analyzing the rendered game image to find what to shoot, like a human. Neither do they have to move a physical object over a desk surface to indicate they need to turn.
In fact, most computer opponents have the deck stacked in their favor. They know the exact location of every player, how tall they are, where the ideal headshot point is for this enemy type/character model, have a gun they can just tell „shoot at this coordinate“ and it‘s an exact hit, etc. So to make the game fair, you now have to approximate the limitations of a human.
So a big part of this „dumbing down“ is necessary simply to create a fair game for opponents that play by different rules.
If you’re already doing that, it is a small step from there to applying a few more modifications for storytelling effect and to make things more entertaining.
Another aspect is the accelerated timeline on which most games run. There are four-hour games where you are a sniper. It takes much longer to learn how to precisely shoot a gun. It takes longer than that to track an animal through a forest. A good game finds the right balance between making the game trivial to beat and having to attend years of police academy.
So it depends on who and what a game is for, whether these choices are good. For example, many big games really aren‘t a single game. They are more several games merged into one, to appeal to the broadest possible audience and make enough money to recover the huge up-front costs for a professionally acted/mition-captured, voiced, scored and textured game.
So you‘ll have players who want to compete with other players, who only care about the battle mechanics. You have others who only want to experience an interactive story, and find the battles between them a chore, and again others who are mainly looking for a crafting game or a resource management- or trade-simulation.
To find the sweet spot here, you have to provide a challenge for each type of player, but you also have to keep it simple enough that the other player types can either ignore those undesirable (for them) mechanics, or complete the game while doing badly in them.
In single-player games, this is often achieved by allowing to prop up deficits in one mechanic with exceptional skill in another (i.e. craft an awesome armor and the fighting becomes easier), or by having difficulty settings that let you turn on/off the parts that irk you.
In multiplayer, that‘s not really possible, so you match up players with similar rank/gear and adjust difficulty for the whole match.
However, these are all fairly "above the table" tricks. Every player gets the "secret double-damage last bullet" and can take advantage of it. Most peoples' intuitive understanding of how likely a "50% hit chance" really is is wrong, and they will be helped by telling them the percentages "translated" into their "language". What, as you say, about "secret" tricks?
Well, if a game was like chess, and you were told one set of rules, but really those were a lie, that would be cheating.
But none of the games in recent articles I've read on game developers' tricks to make you feel better were actually like that. Most were story-heavy games, or games that are famous for invoking a certain mood or feeling. And in that context, displaying 100% mathematically correct health bars makes the game much less exciting.
It's like a magic trick: Players go in expecting to be served an illusion, so I think it's fair to give it to them. It's also like a magic trick in that some people will still want to believe there is actual magic and will be disappointed when they find out about the trick.