Because 'self-determination' doesn't say what is a state, what is not, or what self-determination really means; independence, autonomy, or just taking part in the government. Let's not forget that 'self-determination' didn't exist until a century ago. It's not some sort of nature law, it's a creation. States loving holding claims to 'international law' because it's international law that dictates souvereignity. Yet the irony of it is that the argument was meant to protect small countries from big countries, and didn't really take into account the idea that dictators and other 'bad' regimes would exercise the right to souvereignity to terrorize their people.
All in all, international law is shite and the fact we're still using it is because some people benefit from it. Take away international law and we'll go right back into the post-WW1 situation.
You confuse the act of determination itself and the
right to self-determination. Having a
right to self-determination only means that you can decide your own fate. Nothing more. This right is a derivative of the so-called "natural rights" (I hope you don't actually believe that those rights are natural and not a construction). Indeed, there is no complete freedom if you are oppressed in your national identity.
In practice, this right means that you are allowed to vote and decide, without pressure from the outside. In the Ukrainian situation for example, Crimea had obviously a right to self-determination considering that this region was only gifted to Ukraine a few decades ago when it was still part of the Soviet Union. Since the Soviet Union has disappeared, and since the overwhelming majority of the inhabitants don't speak Ukrainian, they had a right to self-determination. And nobody can challenge that.
This is also true for some regions of what is called Ukraine in which the Ukrainian language is a clear minority. Those population had the right to self-determine themselves, and had much reasons to break with the rest of Ukraine after the crackdown on Russian language and the violent overthrow of Yanu who obtained most of his support in the Eastern regions. Yet the Kievan government considered that this was its territory and denied the right of those populations to self-determination.
If the inhabitants of those regions had had the possibility to self-determine themselves before the war, it's doubtful that they would have chosen independance. They would have asked for more autonomy probably and Ukraine would have remained united. But now considering the amount of destruction and death, I don't think that they would accept anything like that. That's why the right of self-determination is especially important. It's the best way to have people live altogether in peace. Denying this right implies oppression, and oppression often leads to war and separation.
Poll