Try setting up a non-aggression (ie, move freely inside one another's territory whether you like it or not) treaty, and the visit the AI's territory. Literally every time I've gotten inside, I've watched the AI lose units, outposts, and cities to wandering monsters. Not on every turn--but then, the wandering monsters on my territory don't attack me on every turn, either. But sooner or later, it's Rodan faces Tokyo, only without the motion picture cameras to let everyone know what's happening.
For example, in my current game, I've had a dragon revisit one outpost of mine for over 70 turns. It comes, it goes. It hasn't done a thing. If I'd been an AI, how many people here would be screaming foul play?
On the other hand, I've gotten carte blanche to travel through one other AI with whom my kingdom is close. Just upon entering, I saw a stack of spear units first kill some wandering darklings, then get torn apart by a group of four trolls. The trolls then swallowed an outpost. All you have to do is sit still, and watch. Because it won't happen quickly, but it happens all the time.
It's the monster AI: most don't attack right away. They engage in a behavior that changes from turn to turn, rather like the behavior in RPG games of characters under a Chaos spell. I personally think this should be tweaked so the percentage of turns when they focus on attacking whatever is nearest is higher, but that's where things are right now.
This whole subject is a zombie. It has been raised several times before, and killed with statements by Brad and Derek, and with video by players. But then someone feeds it with suspicion, someone else agrees, and suddenly they're buying into something that isn't accurate. The monsters do attack AI players. Watch, and see it happen.