Beehaw has it as World News and US News and those two far predates the ones on lemmy.world.
It seems lemmy.world is just increasingly just all Reddit refugees and they’re copying over subreddits 1:1.
Why are you surprised that Americans are arrogant?
The trouble is, they get away with it. No other country would be arrogant enough to assume that they are more important than the entire world, so the rest of us have got used to if the country isn’t specified, its 'Murica.
It’s because a lot of people migrating from Reddit landed on .world and created or found their familiar named communities and that’s how it was on Reddit - US centric top level type communities.
Is .world the best place for US centric communities? Maybe not just based on nomenclature but since you asked why, that’s the reasoning. It’s in the description of Politics specifically (it even calls out migrating Redditors).