Because the people of California and the people of Oklahoma both agreed to become part of the "United States", and in turn, the existing United States at the ti

Because the people of California and the people of Oklahoma both agreed to become part of the "United States", and in turn, the existing United States at the time also agreed to let them join. And the US Constitution and rules say that the residents of the states can move between them. So "self-determination" does seem like an important part of the reason. There is no reason in theory why, say, Mexico couldn't apply to become part of the United States and the US accept them, but I suspect the reason this kind of thing doesn't happen any more is 1) being a leader of a nation state is much more fun / fulfilling than being governor of a state in the US, so why would the leaders of a country ever agree to it? and 2) the level of welfare spending under existing rules in the US would require a massive wealth transfer to any new state that isn't as developed an economy. Personally, I still think it would be worth it for both the US and any state that wanted to join, but I don't think it will ever happen again.