All of the castles were bought centuries ago! Even if you could buy one now you can't be sure that the real owner wasn't just sleeping for a few decades. They might pop out and eat your face. It's just not workable in this economy!
Any respectable vampire needs to own a castle but I'm smart so I'm waiting for the market to crash. My sire lets me rent a cell in the dungeon, so I'm not like you castleless vermin. I'm working hard, I'm scrubbing blood off the floors, I'm making sure the cobwebs are pristine. It's only been 50 years but eventually I'll be able to afford a room in the tower!