I think people might be failing to consider the AI's plan. If your not playing a conquest game, why invest resources in conquest? What if the AI was gunning for a cultural or science victory? It'd go well for them to just attack and pin down any enemies on their continent. And why invade yours? your assuming that as a tactic, it would do them any good. In a multiplayer game, I wouldn't waste precious gold building a vast navy and invasion force, purely for the sake of it?