It might not be that they are "uniquely ambitious and generous" but rather that the US has a much greater wealth disparity on average, so the rich are much rich

It might not be that they are "uniquely ambitious and generous" but rather that the US has a much greater wealth disparity on average, so the rich are much richer and presumably have a lot more to invest. https://en.wikipedia.org/wiki/Gini_coefficient https://en.wikipedia.org/wiki/Gini_coefficient