America is the only big country in the world that works the way it does. Almost every country is an ethnic monolith.
I'll just disagree a tad there. A lot of the Americas are racially diverse, not just the US. Canada, Brazil, Cuba, just to name a few, have similar ratios of racial demographics to the US (maybe Cuba to a lesser degree, you don't hear of many Asian Cubans).
If we're talking specifically big ones, Brazil still fits the bill. Brazil is not without its own problems, however.