Oh, there is. Several catches. They're usually fixer-uppers or very small, in rough shape, in the middle of nowhere, and a LOT of them are on the historical registry, meaning that trying to repair them would be an uphill fight with local city/municipal councils about what you can and cannot use for repairs, or if you can even DO repairs. I like the IDEA of living in a castle, but it is not practical.
That sounds like a lot of headaches. Government red tape can be rage inducing. Needing repairs is one thing, but needing repairs and the government not letting you do them is another thing entirely. Not worth the hassle. That's probably why there are a fair number of castles that are just left to rot.