It's easier. I go to another UC myself and while I agree with most of what you say, I see more and more people interacting with one another. Hopefully this trend continues.
But I get it. For almost all of our evolutionary past, we were in tribes where everyone looked like us. While we're more open to diversity, it takes a certain type of person to want to go outside the comfort zone.
In the past, this would mean getting eaten by an animal or getting killed by a rival tribe. If you looked different, believed in different things, etc. you are dead.
So being with like-minded people, even if you're liberal or whatever is the remnants of human evolution that we haven't fully gotten over yet.
Besides, it's safer. Easier to relate to people who look like you and think like you.