Castles sounds really cool to Americans, but in Greece (for example), medieval castles everywhere in the countryside. And as others noted, most are in ruins (this is true in Greece too). In some countries, like France and I believe Spain, it was popular at one time in the early modern eta for aristocrats to rehab medieval castles into little lavish palaces...hence why "castle", which was originally a fortification, became something more like what you see at Disneyland.

In Greece, most medieval castles are ruins, and -just like monuments from ancient times- medieval castles are open-air museums owned and controlled by the state. The state maintains them as historical archaeological sites; a few have been partially rebuilt over the years, but again, they're state property. There are a handful of small castles that have somehow remained in private ownership over the years, and a few of those have been rehabbed, but into hotels. The reason they haven't been turned into someone's primary residence, is because -as noted earlier- they're located in the country, outside of cities, so no one with a job in a city can live there and commute. However, the locations are perfect for hotels.