Yes, but there is far more diversity in the US than in other regions, let alone other countries.
For example, there's more diversity south of the Tehachapis than there is in Western Europe. (It would be mean to point out the diversity within 50 miles of Disneyland.)
Are you sure about that? I lived in Pamplona for a year, and I noticed a lot more cultural difference between the local Basques, the French Basques across the border, the Spaniards to the South and the Catalans to the East than I ever saw between LA, NYC and Nebraska.
That's one 50 mile circle. Europe has dozens of similar examples.
Thais, Cambodians, Vietnamese, Chinese, Japanese, Mexicans, Columbians, Armenians, Turks, Iraqis, Israelis, and so on, And yes, I'm ignoring the differences in each and I haven't even mentioned the Europeans.
I think I see now what you're using to define diversity. America certainly does have plenty of immigration, and plenty of "little wherever"s, but really they don't make much of a dent in the Monoculture.
If you're going to argue that Northern Basque communities near Southern Basques communities is diversity, then a "little Korea" next to "little Manila", each with more people, qualifies.
It's not a "monoculture" so much as a dominant one. Yes, one can live within the monoculture, but the ability to make that choice doesn't mean that one could choose to live in a large number of cultures within the US.
And, both monoculture and dominant culture overstates things significantly. Even if one thinks that NYC is one culture, it's not the same as upstate NY's culture, let any of the ones in Texas.
Yes, there are lots of places in the US where one can find very similar cultures, but again, that's a choice. (Ever hear the term "don't Californicate Colorado/Oregon/Arizona"?)
You'd think that folks who claim to value diversity would actually practice it given the choice.
Yes, but there is far more diversity in the US than in other regions, let alone other countries.
For example, there's more diversity south of the Tehachapis than there is in Western Europe. (It would be mean to point out the diversity within 50 miles of Disneyland.)