I think your premise is wrong. Scandinavia isn't as homogeneous as you think, and the UK and US aren't as diverse as you think. I can only assume China isn't as homogeneous as you think either. What it is, is that we are all conditioned to recognize a lot of diversity in what's familiar to us, while communities of strangers seem more or less the same to us.

I remember how relatives from the US visited a central European capital, and were surprised how little diversity they saw. To them Hungarians, Germans, Slovenes, Poles, Russians, Turks, Roma, Bulgarians, Greeks etc seemed all pretty much the same. I remember how I first visited a couple of north American major cities and was surprised how little diversity i saw. To me they seemed all pretty much the same.