Yes, I have, but I didn’t think I needed to write my whole life story to make a point. Regional accents (Massachusetts has a couple distinctive ones! While we’re being condescending, surely you are at least vaguely aware of that) are more likely to be used by people middle-aged or older. Regional media is dead. Regional cuisine is far less distinct than it used to be with many specialties gone.

In 1900 it wouldn’t have been common sense that MA and CA are “supposed to be” similar so the fact you’re throwing that in my face is actually making my point.

I think this sort of proves the point that Europe is significantly more varied though. Even if you only travel to major cities in Europe you notice very significant cultural differences between them. Small pockets of distinct cultures are around the US but there are way fewer of them and they are sparsely distributed in a way that most people don't run into them at all.

For example, in Canada the difference between the Haida people, Inuit people, English Canadians are very apparent and obvious, but the vast vast majority of the country (outside of Quebec and northern NB) is small towns where people speak English, play the same few sports, have the same like 8 chain restaurants, and live in broadly similar property styles. Sure culturally BC is different from Ontario slightly, but having traveled to and lived in both for many years, the differences are not very significant unless I were to seek out specific cultural enclaves, and that is a 4500 km difference.

No argument against the idea that the ~continent of Europe is more heterogeneous than the country of the US.

Just that the MA-CA example is vastly underrepresentative of the variation within the US.

Isn’t it a little odd that we can have culturally very similar places 3000 miles apart unless culture has mostly been nationalized in a couple variations?

I mean, superficially odd perhaps, yes. I can think of a dozen reasons why it makes sense.

Regardless, it is not evidence that the culture is homogeneous in the middle.

There is, of course, a national culture. Again I think we might be drawing the line differently. I look for and enjoy the variations. They are there.

But if you want to paint with a broad brush, you can make lots of generalizations too. It's just less interesting.

What forces would we expect to act on the coasts of the United States but not the interior?

Cosmopolitanism- more multicultural cross pollination due to trade and travel. Wealth disparity is another. The coasts are also richer than ‘flyover country.’ There are isolated places in New Mexico, the Deep South, and Appalachia that are grievously poverty stricken, to give some extreme examples.

If we want to get so nitpicky there are poor and isolated areas along the coasts too. And heartland areas outside of major cities are less able to sustain independent media. Modern institutions like the military shuffle people around to and from these less notable areas and many broad trends do not fit the simplistic narrative you're presenting (for instance, many rural areas have large immigrant populations who work in farms, meat-processing facilities, and similar).