Playing against AIs and Humans are completely different things. Playing with humans will never give the "This is an immersive world"-experience, it's always a competitive thing. Even after having played tons of WC3 and SC2 competitively in the past I have no interest in the multiplayer part of Civ, it's just not what I expect from this game, I want a world that I can manipulate, that I can shape, not a world that is full of other players that I'm competing with. To get a decent challenge in such a world without it just feeling like you're a small fish among Gods (like it's the case in Civ 5) the AI must be decent, too.