Sumer and other Mesopotamian Empires, Egypt, Rome, the Incas, Aztecs, Great Mali... you name it, were ethnically, even racially mixed from early on. The people who lived in territories that they spread out into simply didn't disappear to get replaced, they became part of the empires. Same with the Spanish and British empires in more recent centuries, as well as the United States of America.
China and Iran - the world's two oldest countries - have been religiously, ethnically, and racially mixed since their beginnings. Cyrus the Great didn't become the first - and only Gentile -Messiah of the Judeans because there was an opening for the gig.
As for volunteerism and charity in more diverse communities, for one thing they tend to be larger. The larger the population, the less likely for individuals to want to get directly involved. They also tend to be more transitional - waves of ever newer migrates keep arriving, they settle in, then their children radiate outwards to other communities. They might have caught some flack upon arrival from more established inhabitants as well, and that tends to instill something of a lesser drive for more altruistic endeavors regarding the larger population. There's also the notion that there already are sizable established charitable and other organizations geared to helping, so it might be perceived that there's less need to.
But yeah, you compare 20 million people in metropolitan New York to some unknown Hamlet in Scotland where people can trace their roots back to Neolithic times, and yes, there will be something of a pronounced difference in community involvement. Even simple things like PTA meetings and the homecoming HS game. People in small towns are more likely to do PTA, and they even have parades for the homecoming game.
My high school, Fordham Prep, in comparison, couldn't pay people to go to the annual homecoming game against Xavier. Sure, we'd have a rally the day before, and they'd beg students to go, and some do go, but the vast majority couldn't be bothered.
Now if you look at Fordham, the students live in a widespread area, some commuting by rail from farther out in Westchester, as well as Queens, etc. That's a bit of a trek for a measly game, and people have busier lives that don't have a need for football culture becoming as big as it does in Tumbleweed towns.
Meanwhile, as for the dedicated citizenry in smaller towns, they're basically moving out. Economic opportunities are great if....
tbc
