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).

Memmy’s on the App Store now. It does still have a TestFlight if there are any spots available but the app store’s version is currently up to date.

lemmy.ml is down right now. You’re going to have to wait until it’s back up to ask them - this question/answer is just local to lemm.ee until it comes back up and can appropriately federate.