It’s an acknowledgement, that the best way to organize the world’s population, which consists of mutually-exclusive cultures, is to separate those different cultures into separate Nations.
Think of your neighborhood. Everyone lives in their own house, where they have their own resources and freedom to live according to the way they want, without infringing on others. The houses, in essence, are nations. You can have neighbors with different cultures, and get along because at least you don’t conflict.
Now. Imagine that instead, these neighbors are forced to live under the same roof, and have to share those resources. You can imagine the inevitable conflicts that will arise. That is multiculturalism.
“Think of your neighborhood. Everyone lives in their own house”
Go a step further. Think of every species of animal or plant. There are many ‘copies’ of each species, not one giant instance of the species.
Globalism hasn’t worked for life itself.