It has everything to do with "human rights" and "democracy"! No one said its simple, but a generic „its complex“ is not a proper refutation. What exactly makes this impossible? Countries form along ethnic boundaries when permitted to do so, and it is then that they are most peaceful and stable. Otherwise you will end up with suppressed minorities and ultimately war. Look at what happened upon decolonization and the fall of Yugoslavia.

People are more peaceful as neighbors than flat mates.