Why do celebrities tend to date/marry other celebrities?

It seems nowadays (maybe always) that celebrities will only date other celebrities. It’s like “celebrity” has become its own breed of human and god forbid their offspring be mutts.

I realize there are exceptions, but generally speaking, why do you think this is? Is it so they are guaranteed a hot, known date for the red carpet? Is it because their social circles are so small that’s really the only people they meet? Or maybe it’s for the compassion, famous people know how other famous people feel…