Because people who are tribalistic to a fault spend your entire life preaching how evil the very idea of differences between people is (only applies for whites, who exist to get diluted), which eventually manages to infect even those who recognize it.

That's why somehow the "literally illegal aliens" question still schisms the Right, because "well, but they work hard and are super Catholic so they should be allowed to stay!" is somehow a very parroted viewpoint. Heck outside of niche forums like this, the very word "segregation" gets you attacked.