This cuts both ways. See what happens in states where the border does not respect these local realities, for example where you have sunni led by shia or the other way around. Or look at Africa where Western powers drew lines on maps without respecting ethnicity.

Nation state is one form of decentralization, letting people govern themselves how they want.

Though experiment: imagine that there was a single global state, with one set of laws and institutions. How do you think that would work out?