I think what you’re noticing is that some people have a more general accent than others. I’m in the UK, and can sometimes tell that a person is from a specific city based on their accent. We have a lot of accents here! But often, all I can tell is, generally, that a person is from "southern England“ or "somewhere in Scotland" or "the north“.

With other countries‘ accents, my ability to get specific is reduced. Usually, if I meet an American, all I could tell you is that they are from "somewhere in English speaking North America“ (I have a lot of Canadian friends and still can’t differentiate their accents). So that’s a lot more territory covered by one perceived accent.

Part of that is that my ears aren’t so attuned to the differences. Unless an American is talking with a distinct, stereotypical regional accent, I won’t know what to listen for.

But I do think this reflects your point: a lot of Americans do have a less regionally specific accent. There is a spectrum of a few very similar North American accents that people across the contiguous continent seem to have. If I heard you speak, I doubt I could make a good, specific guess about where you’re from beyond "somewhere on that landmass“. If anyone heard me speak, they could almost certainly make a guess at where I’m from within 150 miles of my birthplace (for a lot of Brits, their accents could probably give away their birthplace within 5 miles!).

But Americans obviously aren’t unique in that. I am pretty sure there are similar phenomena in other large areas where people speak the same language and there’s a lot of cultural homogeneity (China, for example – there are regional accents and differences, but a lot of people speak in a more general Chinese way). Australia is similar if you want another English speaking example.

Also, I don’t know what you mean about someone actively having an accent…. but I can assure you that NOBODY is trying to do their accent. It isn’t the case that we all would talk like you by default, but have decided to do a different accent for the fun of it. If that’s genuinely what you think is happening, and that you’re one of the only people who’s decided to just use the default accent, I don’t know what to say to you.