All of my life I've felt at a distance from others who exhibit typical American social habits, and have actually got along better with people from various other cultures such as eastern Europe. (There are a lot of first and second generation immigrants from eastern Europe where I live in the US.)
I have always been perceived as rude and blunt, and in recent years those with typically American social attitudes have even begun pathologizing behaviors like mine as being "on the spectrum".
As to why I am like this was a mystery to me in childhood. I acted just like my family and extended did! Everything was normal at home, and then I would treat someone at school like I would treat a sibling and it would result in arched eyebrows.
Turns out my family's attitudes towards socializing were strongly influenced by my mother, since she was a stay at home parent. And she came from a part of the US made up of small agricultural communities that were mostly German and Dutch in origin. These small communities had transplanted their national cultures in little bubbles all over the US, and by dint of mostly isolation from the broader culture had maintained those mores for centuries.
It was only when my mother had moved away to the city that this culture shock occurred, about 150 years after immigration.
So yes, there is a lot of cultural variation in the US, probably not as much as in Europe, but it exists. There are subcultures within the US of various vintages that exist alongside the mainstream culture that have all learned how to be different and coexist at the same time.
Decades later, that's clearly an exaggeration or just plain wrong, but it is certainly true that people do underestimate cultural differences within the US, and overestimate them within Europe.
The language barriers makes Europe much more culturally diverse, but also makes that diversity much harder to discover since you can't fully understand a culture without learning its language.
It is more like the cultural difference between Canada and Mexico, you don't have such a wide spread within USA. You have some parts that is more like Canadians, and some parts more like Mexicans, but Mexico to Canada is still a bigger cultural leap than anything within USA.
The opposite is true in the USA: nearly everyone speaking English to each other, watching the same TV shows, driving the same cars, eating the same breakfast cereals - easy to believe they must all be culturally very similar, when in fact they are not.
But we also know that there are much bigger differences if you cross many national borders and language barriers by going to the other side of Europe, they too will have those micro differences but when going there you also get many major macro differences.