That surprises you. You can get that different ways. The Japanese do it by all being the same culture, except for a few Koreans and Ainu. We did it once by having one dominant culture, with reasonable standards of civilization, and imposing them on everybody else by grim determination and self-confidence. It was the right thing to do.
Society needs to have the gumption to enforce civilization.
It's like being a parent. Thing is, to impose civilization you gotta know it is civilization. It doesn't do to figure it might be, but you aren't sure, and maybe everything's beautiful in its own way. You have to mean it. That's the problem, though.