It's not that Americans don't care about anyone else on the planet (I doubt they care less than people in Western European countries). It's that Americans aren't willing to bind their sovereign freedom of action. The idea of "human citizen[ship]" is anathama to American thought, because citizenship implies reciprocal rights and obligations, and Americans viscerally hate the idea of being obligated or beholden to anyone else. Hell, a lot of Americans think that the arrow should be pointed in the opposite direction: more state sovereignty (i.e. people within states should be less obligated to rules made by the federal government).