I am curious of this as well. Maybe we are choosing some settings that is putting them into a dormant mode?
I have only played 2 games, to try out both fractions, and in both games I am seeing no AI challenges. No city attacks, no city building, and no NPC's roaming the map after the first 50-100 turns. I even added 2 more AI to the default choices of a Large map on the 2nd game (not finished yet).
This is great for learning how to play, but I think others have mentioned that their towns were attacked and the AI was really agressive. So, no idea.