A game I play right now does have an AI that will step in for you if you are AFK for too long. Granted it isn't a universal AI that will work any game.
That's not AI. It's an AFK feature that idles your player so it isn't killed. It can't really do anything while you're away, and it's annoying to other players if you use it all the time just to level up an avatar's resources by having it survive a long time on AFK.
Players also get pissed, AFK themselves, then leave, just to irritate the players they are mad at. The AFK'ed player is still there, but no one can kill it. It might really screw up the opposing team in a multiplayer team game.
If you're away too long, the server might make your player start participating again, but that's not going to help you much unless the server knows what you need to do. Players often have jobs and specialized abilities. The server won't be able to tailor what you do to help the team.
AI requires observation, analysis, and decision making on top of accurate execution to win. AFK isn't that.