The AI never seems to fancy its chances, in field battles or sieges. I seldom get it attacking me (when I'd rather it didn't) unless it has at least 2:1 advantage, usually more like Sun Tzu-esque 3:1.

To get it to attack you, you have to deliberately under-garrison.

Personally, I'd love to do some siege defence battles. You get to chew up a lot of their men without lifting a finger.