Now, I have a theory on this.  I play on dense monster settings and there are monsters literally everywhere in my game, even about mid game.  So anyway, I've just started meeting AIs and there are unclaimed equipment and empty monster lairs everywhere.

I just captured a screen shot, I should have got one earlier when there was literally like 4 or 5 unclaimed goodies in one screen:

Now, my theory is that the AI is too busy fighting monsters to worry about picking up equipment.  Or maybe they defeated mosters with troops instead of heroes.  However, monsters aside,  I think the AI really should give priority to picking up some of these caches, especially from higher level lairs.  Up until this point, I was struggling to get good equipment for my heros, which was fun!  I was really enjoying that good equipment mattered more.  But once I met the AI, I was getting awesome armor, super powerful swords, etc. just from unclaimed goodies.