More Words, Deeper Hole

Answer one: third largest country in the world by population, which provides a lot of room for subgroups of self-sustaining oddity (and worse).

Answer two: the United States is full of cultural narratives and bits of cultural mythology that are at worst harmlessly eccentric on the personal level but which scale into "awful" when applied across the whole country. Libertarianism is an excellent example. It makes very little practical difference whether an individual person you know is a libertarian provided that they're not full-blown objectivists who apply those principles to every social interaction (which is exceptionally rare, even among libertarians). But application of the philosophy at scale quickly becomes toxic.

Basically, the US is full of ideas that don't scale to 320 million people and nonetheless have been scaled that way. Other countries sometimes do better because they lack those principles but, honestly, also sometimes do better because, despite having their own oddities, they've never had to scale those oddities to 320 million people and had them break down in the same way.