There’s two different issues. In the US, demographics is highly correlated with other factors. Even the most progressive parents will self-segregate to get away from these issues (under the pretext of finding a “good school district”), which in practice means the suburbs.

Apart from that, cultural differences become much more salient when you’re a parent. Because culture is how we transmit our values—what we believe about how to live a good and successful life—to our children. Culture is also highly correlated with demographics and also is a source of conflict. My kids attend a private school with affluent white and black kids (about 2/3 and 1/3). The neighboring supermajority-black county is richer than our supermajority-white county, so there are no socioeconomic divides. But there’s quite a bit of racial conflict nonetheless. A couple of kids in my daughter’s friend group wanted to kick her out of a running group chat because they wanted to make it black-only (she looks fully south asian though she’s half white). My daughter is far more accepted by her white friends who aren’t socialized from an early age to have a sense of racial identity/group solidarity.

White/asian places like Brookline have cultural problems as well. My high school was about 30% asian, the remainder white (all affluent and educated). There was a lot of conflict over cheating and the school being a “pressure cooker” that boiled down to conflict between the asian immigrant proclivity of academically grinding to get ahead and the WASP mentality of raising “well rounded” students with diverse interests. Studies show that white families move out of school districts as asian families move in: https://www.the74million.org/article/fear-of-competition-res.... This is true even though, statistically, asians are more affluent than whites, much less likely to be arrested or imprisoned, etc.

Parenthood is where the rubber really hits the road in terms of any sociological preconceptions people may have. It’s the time when you’re faced with applying those ideas in practice in a situation that will have consequences for what you hold most dear.

It also doesn't help that any perceived error in raising a child results in a phone call to CPS (by literally anyone) and possibly you and your children being terrorized, and much of this perception is cultural. The more cultural diversity, the more chance you come across that single person that doesn't approve of the way you do it.

This has happened over as little as a kid walking home alone or playing at the park while the mom is at an interview.

I've partially chosen to raise my family in a place with people like me because I know the neighbor kids drive trucks, work in construction, explore the backcountry on dirt bikes, etc from a very young age and I don't have to worry about the local nazi mom sending the child snatchers for the sin of a child having some independence.

Yes, that’s yet another dimension! My wife is a free-range Oregonian, and that’s a reason we couldn’t raise our kids in DC where most parents would love to give their kids an ankle monitor.