Part of the yell-talking is trying to hear each other over the stupidly loud music and TVs in our restaurants. But I've noticed that even abroad Americans are louder than most nationalities when eating out.
I grew up listening to self-hating Americans complaining how American tourists are the worst. Then I worked in a hostel in Prague and realized how loud everyone is, and how everyone swears there's some nationality that's the worst whether it's Brazilians, the British, or the Chinese. They're never the loud obnoxious ones as they sit there sharing their bigotry out loud. It's always some other country.
The only constant is that nobody seems content just taking the upper hand of keeping their stereotypes to themselves. Everyone acts like they've lived 5 years in every country and somehow know how it all crumbles. In reality maybe they've lived in two countries and have had day-visits elsewhere. Hardly grounds for being a master of the nations.
Like someone swearing that <some city> drivers are the worst. Almost guaranteed that they grew up there.
American tourists didn't even have a bad rap where I worked in Prague because you just didn't see them compared to the daily horde of Europeans. By the time you work in the hospitality industry for a few seasonal cycles, you get burned out by the exact same conversations, like people arguing over which nation tourists suck the most. Turns out it's probably just the one you see the most of. In our case, the British. ;)
These days it's on HN where people will post the template: "I took a few day trip to <place> and it's insane how <judgmental thing that people seem to do everywhere>."
Speaking of the national stereotypes and loudness, just put together more than 2 Italian tourists and they're guaranteed to be the loudest in the room, they're just unbeatable... but they do it with a lot of charm, I have to admit...
There are cultures in the world that are more obnoxious and more "impolite" than American in my humble opinion (and I'm not implying anything about Italian culture here). However, I think the distinction of noise in America is that it's a cheap way to make people excited, and it wears people out over time.