It's not a left/right thing, nationalism has obviously been a potent political force among many groups of people over the years, but there has never been a situation where you can make a clean break without causing a boatload of new problems to arise. To use your example, sure it's easy to say that the Kurds should have their own state, but the reality on the ground is that they are split up among four different countries in an extremely volatile part of the world, and much of the territory that they claim is shared with other ethnic groups that aren't exactly on board with Kurdish nationalist aspirations. And probably most importantly, there isn't really any kind of unified Kurdish leadership, and there is a complex network of rival parties all vying for power, often prioritizing internal politics over external threats. Trying to forcibly carve out a Kurdish homeland in the current political climate, especially one backed by Western interests, would be a catastrophic disaster that would create many more problems than it would solve. And you can go up and down the list of nationalist movements and you'll find similar problems with every last one.