I could be convinced of your position if you can name the common elements that justify the umbrella. It becomes even stronger if those common elements are the rotten ones.
Anecdotally: I married someone from a very different (USA) culture than my own, and their family feels as different from my own as some people that I’ve met from other countries, so that’s partly why I answer that question the way that I do.
I would say our business and media cultures are most homogeneous. No matter where you are in the US, a business conference feels the same. A meeting with clients feels the same. The same toxic personalities rise to the top. I’ve never been surprised by a director or CEO.
No matter where you are in the US, you turn on the TV, and even if it’s local news, you get the same predictable messaging. We’re all obsessed with appearance and status, even within our subcultures and across the political spectrum. Superficiality.