As a Virginian living in Sweden, I think it's actually true that that the US is more culturally homogeneous than Europe. Someone from the East Coast and the West Coast still watches the same TV shows, goes to the same restaurants, and votes for the same president. It's hard to tell the differences in accent between the West Coast and the East Coast.

There's probably a bigger cultural difference between Richmond, VA and Lynchburg, VA (home of Liberty University), than there is between Richmond and Seattle.

In Europe, you can go 100 miles and find people who watch different shows, have different political parties, and speak an entirely different language.

The US was founded all roughly at the same time under the same government, with minor differences based on immigration and former colonial history. In contrast, Europe is dozens of different countries with widely different histories and language groups.

Other countries, like Russia and China probably have more cultural diversity than the US due to their languages and histories, but not as much as the EU.

One of the goals of the EU is to bridge these gaps between countries so that business can be conducted across political and language barriers, to make Europe have as much unified strength as the US. The EU has a larger population than the US, and nearly as much GDP, but you couldn't tell on the global stage, because it's not a unified force.