Very few countries have a monolithic culture, that doesn't mean they are multicultural, America has an adaptable, assimilationist culture, but it is not multicultural.
Well what countries would you call multicultural then? In my view having a bunch of different ethnicities living in a country together yet being closed off and seperate unable to even communicate together isn't quite multiculturalism, its something else that I cannot think of a word for right now. A perfect example of monolithic country in my opinion is Japan, almost everyone there has the same biological and cultural background.
America is an "unofficial" multicultural country because there are plenty of people from all different parts of the globe and all those people have their own ancestral culture/language/music/values/religion all Americans share the same basic modern "American" culture but each individual family has its own culture they "practice" when they're at home or when their with people who share the same ethnic/ancestral nationality