I mean the other 2 countries, Canada and Mexico, how similar are both of them to United States?? Both countries have a similar economy and democracy etc, and I think those two countries share things like supermarkets, stores, etc. I suppose the cultural differences are not a lot, that is very nice.
European cultures had time to develop before travel was easy, so in practice they were much further apart in terms of culture spread. The territorial size argument here doesn’t work.