Every aspect of human society exists because we, as a society, believe in them, including society itself. The very tools we use to measure the world beyond man are human fabrications. However, the maintenance of a social model does not necessarily depend on all its members believing in or agreeing with it, only on continuing to work for it. Cultural, social, and physical constraints exist and are very real. For example, it was common a few years ago in the "Free Palestine" online community to say that Israel is not real, but this statement has never stopped any of Israel's oppressive actions from happening. Understanding that all systems are fabricated is a fundamental step towards the possibility of replacing them with better systems, but for this to happen, realization needs to evolve into organized action: the only tool capable of changing the world. And yes, it is only possible to replace one system with another, and it is not possible to live without a system, because what makes us human is precisely this characteristic: we created the social system to overcome the evolutionary system.
And to rationalize the world we live in, we create rules to legitimize our other creations. We can use any factors to generate these rules, but to avoid chaos, we agree, in materialism, to use historical, cultural, and economic factors to justify the control of a territory by a nation state. Considering these factors, in a comparative sense, the control of the United States over any of its constituent territories is much more illegitimate than the control of Tibet by China. Does this mean that we should dismember the United States and return its territories to their original owners? No, it means that someone who believes that China should grant independence to Tibet should also advocate for the dismemberment of the United States. Since in this case the decision came from the United States government, which, I imagine, has no intention whatsoever of dismembering the United States, we can conclude that the only motivation for this is to antagonize China, and it does not stem from a concern for the right of peoples to self-determination.