Because it mostly is. The vast swaths of the inland US are cultural black holes of consumption. There is simply not enough population density, diversity, or opportunity to generate anything meaningful. It's probably hard for Europeans to understand that in the US, I can get in a car, drive a thousand miles in any direction to a new city, get out, and find the exact identical built environment around me, inhabited by the exact same people, speaking the exact same language and accent, wearing the same clothes, eating the same food, driving the same cars, listening to the same music, and practicing the same religion.