I'm guessing that it's more like people are getting them stolen regularly. What I don't get is why the bottom of the pyramid (cf other branch of this thread) doesn't fill up.
Because people who possess stolen bikes are just as vulnerable to bike thrives as everyone else. If only non-stolen bikes got stolen, the pool of people willing to buy stolen bikes would probably not support any significant level of theft for very long, but that's.not how it works.