>>510907019 (OP)the America that got itself lordship over the world was "multicultural" but it was EUROPEAN culture in specific, and this culture was often modified to fit into American society. Most euro cultures can share ideas and attitudes with each other, they are not so different as to make that impossible.
The reasons that we cannot create a truly multicultural society are as follows
1. society simply does not work without assimilation of some sort, everyone has to follow the local norms not the norms of their own culture
2. many cultures and races do not mix or have ingrained practices (ie FGM) that are incompatible with other cultures morality.
3. a society with no mostly unified culture is not a unified society
I'll also echo the point that earlier immigrants kept their cultural practices but came here to be Americans and to at least partially integrate into our culture.