I don’t think you understand how much of a difference it makes to speak another language.

You are far less likely to marry someone with a different language or move somewhere with a different language. From what I read on the internet, Americans move between states several times in their lives. That is a very equalising factor for the culture and general way of life. There’s much more exchange and assimilation if there is constant mixing.

How a French person and a German person view life, work and public conduct can differ greatly. And the differences won’t be equalised soon, because of the different languages and therefore less mixing.

Surely the states have differences between them, but you have one traditional Thanksgiving dinner nationwide for example.

What an Italian considers a traditional Christmas meal would not be considered “christmassy” in Finland.

On the political sphere, there are constitutional monarchies, presidential republics, parliamentarian republics, oligarchies and dictatorships in Europe. Personal freedom and safety might differ greatly. But even within the EU, where personal rights are more or less equal there are still so many cultural differences. At what age you move out, how prestigious it is to go to university, if cooking at home is a great value.

Even architectural trends are different in each country.

Not to diminish the diversity in the US, but two different countries is just another level.