Private cities are just another exclusive thing for the wealthy class
Discussion
Do they have to be? Perhaps they can serve as examples for the rest of the world. Hopefully they can even "trickle down" as it were, the way the wealthy often get access to new technologies and things before they become more widespread.
The best eventually does trickle down but after it’s all been watered down, completely changed (often not better), or little remaining for the rest to fight over.