It's possible your empire is on very defensible terrain that the AI is failing to take into account and thus they don't build enough to attack you. I've noticed some AI will attack you and sue for peace with generous deals as soon as their main force is defeated. It's actually a nice in-between feature between the AI being passive and being overly agressive. The more warmongering AI will tend to be like this. They will keep testing your defenses until you grow so powerful they dare not DoW. Otherwise, they can DoW and be friendly right after. It's programmed in behavior that works on most maps. Again without knowing your map types and location it's hard to tell. But I think some code for the AI to look at how your cities are placed and to plan an attack around it might be something to put on a wishlist.