I think part of the issue is that AirBnb is probably not very good at catching a banned user making a new account.

I don't think they ask for users' IDs. Presumably anyone could sign up with a fake name and a prepaid credit card, act wild, get banned, and repeat the process.

Determining the identity of an online user can be pretty hard. AirBnB doesn't want to add verification steps that will cause attrition in the rental pipeline, so they'd rather do nothing than do a little and be seen as failing at it.