First of all , this is an observation of facts in the reality we live in. I agree with the OP argument that these things are intrinsically linked when we just observe the facts without trying to impose our beliefs on them. If we accept that, we can try to theorize about why that may be. My theory is that people are nice when they live among others who look and behave like themselves because that what humans did for almost their whole existence as a species. Archeological evidence suggests that if you met a stranger, someone not from your tribe, chances were high they would be a danger, not an opportunity to make a new friend. So humans seem to have evolved to not trust outsiders, not just strangers. In a small society, conformity is highly valued because doing things without understanding its consequences could often end up in catastrophe. People didn’t understand the world, and other tribes, well enough to have any confidence in whether a new behavior would be beneficial or not. So we end up with humans who trust and depend on others from their own tribe for everything, are highly distrustful of outsiders, and not looking like yourself is the greatest sign that someone is an outsider, and who value and demand conformity as a matter of survival. The world of today breaks some of these expectations in some ways, but they are still mostly accurate in my opinion. People who are more open minded tend to be so because they live in fairly cosmopolitan areas, but those also tend to be bigger cities where you don’t really know most people and where trusting others becomes harder, especially when many people have completely different backgrounds than you.