I do UI/UX/HCI (both full-time and freelance)... it's been my experience that they're hard to find because almost no one starts out in UI. You're either a graphic designer or a coder who moves towards designing interfaces.

I think that's mainly because very few university programs with a full HCI track in their comp sci (or ID or graphic design) programs.

In general, it seems like there is a shortage- I'm constantly asked if a I know of any good UI designers who are looking for a job.