They decided to sacrifice verisimilitude for upholding what I call Tails's Law: A game that features AI companions should never allow AI stupidity to severely compromise the player's ability to progress. I call it that because back in the time of Sonic the Hedgehog 2, before the franchise shat the bed, the developers at Sega were canny enough to make "Tails" effectively invincible, as well as make him always follow the player and "catch up" with his helicopter tails -- so that the player need not feel responsible for escorting a character with wonky AI, nor guilty for leaving him behind or accidentally imperiling him. If the actions of an AI character can significantly impede your progress, then you've lost control and that's a frustrating situation to put a player in.

We all know what happens when this rule is not followed. See: Daikatana. Another example: the companion Lydia from Skyrim is basically programmed without any idea of stealth, subtlety, or nuance in combat situations and routinely charges headfirst into battle, putting herself at great risk of getting killed as well as compromising your position.

Generally in The Last of Us, a companion character being killed is a game over condition. If enemies were sensitive to their presence during the stealth bits, they would either have to use the environment and available cover absolutely perfectly, or they would put the player in jeopardy of a game-over without them doing anything to warrant it. The player already has enough to think about. Having to worry about what Ellie is doing and whether she fucks off someplace, attracts a Clicker, and gets bitten while not being able to do anything useful about it is a burden the player can't handle.

At least you can order Lydia around, or send her home. No such option exists with the TLOU companions, and you're stuck with them. I forgive Naughty Dog for putting gameplay above absolute realism.

>just look how Ellie used to hide in plain view of enemy humans and zombies

I loved TLoU, so I might be biased, but I tend to be pretty forgiving of companion weirdness in games -- I can appreciate how hard it is to make that work.

Fallout 4 has similar problems, but they've at least made it harder for your dumb old companion to ruin your life -- before, Dogmeat (a companion German Shepherd) would just wander through tripwires and lasers and stuff. Now, they still act a little wonky, but they don't call attention to YOU while they're doing it. That was pretty annoying.