Being a homogeneous ethnostate does wonders for the social cohesion that allows for a society like Japan to work.
Plenty? Name five, other than isolated microstate islands. I can think of very few modern homogeneous ethostates.
The key point being that ethnically homogenous societies tend to work well. It isn't guaranteed but it does contribute because everyone is the same or similar. Similar ideals, similar goals and so on. The feeling one gets in these societies is stifling. We have nice things but they are for us, not for you.
Someone from BC and someone from Texas actually have a lot in common - both are from immigrant societies and both speak the same language. Can't say that for someone from Kerala, India and Punjab, also in India.