Some people have more money than brains. The amount of money it would take to build and maintain something like that. Why not just buy a private island in a 3rd world Country like Belize. The government isn't exactly ever going to stop by and check in on your private island in those places.
I can think of a few politicians who might. Obviously this is for the guy who has everything and now wants to be a king.
They can just build it as a floating oil rig and be totally self sufficient......and create revenue by selling oil to those evil oil companies......