Fun fact: Brazilian Italians came from northern Italy (venesians, lombardi's, etc) before the 1900s and American Italians came from southern Italy (campanha, calabria, scicily, etc) after the 1900s. Because of the latin culture and language, Italians fully integrated with brazilians but were fully segregated in the US. Today, they couldn't be more different to each other. So much so, that Brazilian Italians are not that big in pasta because they used to eat rice and polenta with just a sprinkle of pasta recipes (just the famous ones, like bolognese) before coming to Brazil.
Discussion
People like to call the U.S. a melting pot but we aren't nearly as melty as South America. I blame protestantism. They mainly didn't mix races. They just pushed the natives out. If you ask where all the natives are in North America you'd send people to a reservation or something.
In South America the Catholics just treated the natives like people, baptized them, married them, ordained them to the priesthood, made them governors and bishops. If you ask where the natives are, it is just everyone.
The few places with tons of intermarriage in North America are places like Quebec and New Orleans, places that were predominantly erm Catholic.