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).
.us is reserved for government's sites by social convention. Someone can have a private .us site, but it's a bit odd that they didn't just use the .com.