Billionaires can go wherever they want. They like places like NYC because we have everyone and everything.
When you have a lot of money it becomes hard to find ways to spend it and big cities have the services and goods that make money do something. In a smaller town or country you might not be able to find things to do with all that money (people with services and skills) that make it worth your while. You can find these things in NYC.
They are addicted.