I've never had trouble with this. The AI seems to be doing a good job of trying to wear me out when I've seen it laying siege to towns, but I've seen it building siege equipment all the time - the only thing about EB and AI sieges that is unfortunate is that the AI doesn't realize it is going to get a penalty over time if it campaigns in enemy territory, and sometimes they are quite famished by the end of the siege if it takes a while.