> Of course Americans aren't constantly running into people from other countries/cultures
Really? Not to sound like a prick but you must live in the middle of nowhere. I live in LA (santa monica, to be exact). I live next to an eastern European deli, kosher jewish, multiple Chinese restaurants, sushi, ramen, and yaki joints, thai, Peruvian, Mexican, Korean, Caribbean/Jamaican/Cuban Vietnamese, French, German, Ethiopian, Greek, Italian, "middle eastern", persian, mediterranean, russian restaurants / groceries / communities / -owned businesses/ the list goes on and on and on. And of course good ol' american BBQ, burgers and beer, standard fast food, ice cream, southern soul food, tex mex, east coast seafood, southern creole, ...
Many people come here from around the world to work in professional jobs, also. And many students and faculty at UCLA, LMU, USC, SMC, etc, etc.
It's just that many of those people all speak English, can act 100% "American" when they need to, and their cultures have become a seamless part of American culture. This is something else many non-Americans don't understand (and Americans don't see even though it's right under their noses).
In fact, if you count the languages that "Americans" including 1st and 2nd generation immigrants speak, it's vast.
Ugly truth: It's just that a lot of Europeans don't count asians, hispanics, blacks, as Americans, so you hear this crap all the time on the internet due to racism and stereotypes of what America is and isn't.