I get what you're saying, and surely the game can isolate each enemy's AI (not in the new sense of GenAI but of the decision making process) from knowing where the character is, but it sure as shit doesn't feel that way in some games where the enemy — as you so succinctly put it, acts like a parent pretending not to see their child who is hiding. Except some of these games don't act like it doesn't know where you are.

But on the other hand — and please don't think I'm trying to move goalposts here, that isn't my intention — if I'm walking up behind you, you probably know it. Skyrim is a good example of this. With no sneak skill, your best efforts mean jack shit since you'll always be spotted. But with Sneak at 100, you can literally walk right by a guy looking for you and he won't see you. Neither extreme makes any sense. So if a game is good at letting you sneak around, I think it's letting you cheat somehow (even though that's the game). If it's bad at letting you sneak, that's a flaw and my character better be able to fight, so I stand a chance.

Deus Ex, much as I love it, had a shit stealth system. Deus Ex 1 had the same stealth system as Legend of Zelda: Phantom Hourglass. I'm talking about the castles where the guards are on the lookout for you. (I think a lot of Zelda games had this mechanic.) Just dumb bots that move in one direction at a time and have a specified cone of vision. If you enter it, they hunt you down until you die (or you respawn at the beginning). A lot of games that do this have systems that let you escape. Break their line of sight and stay quiet and out of sight. But they're still going to hunt you down and that's not unrealistic.

It's fine in Zelda because Zelda is a silly little fantasy game. The more serious a game takes itself, the more I'm gonna dump on that stealth system. In Deus Ex, you had really high stakes. People aren't gonna let you waltz by because you have plot armour and you're the hero.