Part of it is politics. Totally correct about everything in common, and yet in the multicultural fabric we call society, politics could be vastly different:
Neighbour 1 cares about Trump, neighbour 2 about Ukraine, neighbour 3 is focused on Palestine, neighbour 5 about public transit, while I might not care about any of those. All of them are going to seek like-minded people who are unlikely to be their next door neighbours. It wasn't like this in the past, where economic mobility was relatively limited.
Multiculturalism coupled with economic mobility means often neighbours and you don't really have much in common. As an example my next door neighbour: He's a major, I'm in the sciences. We travel in different circles. I have a dog, he doesn't like pets. We both have kids but they are of different ages, don't go to the same schools and basically don't know each other. We met a few times then realized that we have very little in common and stopped interacting. There's nothing binding us beyond a shared geography.