All of those are mostly because any other policy would result in companion-less runs of the game. AI just isn't there yet to read and react to the situation in a way that should take such outcomes in to consideration, and arguably that effort might never be something custom tailored to a game, it'd have to be off the shelf with maybe just some tuning and/or helper hints.
Reading the player's intent would probably still be a very difficult problem, so even with the perfect knowledge of how to interact in the world they'd probably not act and react to changes in the player's intent anywhere near as well.
Fortunately they tend to not just immediately go Leeroy Jenkins as soon as an enemy somewhere in the far distance goes active.
Skyrim was/is awful about that. I'd be sneaking along through a tomb, and then Lydia screams "I am sworn to carry your burdens!" and charges in, waking up a dozen draughir.
Billions of dollars of programming time can be spent on self driving cars, not so for your NPCs.
When large amounts of programming time is spent on game ai it does end up "self driving" at an adequate level, see Google's sc2
bot, Google's older arcade game bot's, and openai's dota bot. It's just not worth it investing that amount of time into most games.
Reading the player's intent would probably still be a very difficult problem, so even with the perfect knowledge of how to interact in the world they'd probably not act and react to changes in the player's intent anywhere near as well.