I mentioned this earlier, but I also think the US is sort of a popular (and in some ways, easy, thanks to our nonsense and our openness about our nonsense) target. And it is like someone insulting a family member. You may know that Uncle Bob is arrogant and pushy and sometimes obnoxious. But when someone else says it, you still feel defensive. So when people pick on the USA, it can bring out the, "fuck no! We rock!" responses, even from people who otherwise acknowledge the country's shortcomings.
I also think the point about the assholes standing out (and the resulting confirmation bias among those who already think Americans are load and arrogant and Whatever) is a good one. If I'm just quietly and respectfully sitting in the corner, no one says, "Oh look, an American who is respectful and well-behaved." They have no idea I'm an American.
I certainly don't think I'm a worldly expert, but I can see the good things, and the negative things about other places I lived. Germans felt sort of cold and unfriendly initially (though they were not, and that was just my perception due to my US sensibilities). If you smiled at them on the street, they looked at you like they were addled. There was no "polite friendliness". In Japan, I struggled with the "can't say no" vibe, which Japanese friends openly admitted. It was even mentioned in our cultural indoc class, taught by Japanese people. Basically, they almost never say no. If you issue an invite, it is "we will see what happens" even if they know it is absolutely "no".
And yet the Japanese are some of the most kind, thoughtful, honest people. I adore the connections I was fortunate enough to make, and respect so many things about them. (While also seeing the things I would struggle with if I were to move to Japan permanently.) And the Germans were warm and inviting in their own way, just not on a surface level. Good, and bad. Like everywhere. But if someone starts harping on how Americans are Bad Thing and Other Bad Thing, I do have the instinct to point out the good (and only the good) because the it feels like a balancing of their perspective.