US territories are a rather different situation because they are not sovereign entities in the way that US states are. US territories are essentially land owned and administered by the federal government so the federal government is free to acquire and dispose of them as it wishes. But the states are sovereigns in their own right that have entered into a compact vis-a-vis the federal government and the other states. Because the sovereignty of the federal government emerges from this compact of the collection of individual states, the federal government generally does not have the authority to modify the terms of the compact without the assent of the states.
I think that's a very peculiar definition of sovereign. Would most not agree that if a state cannot enter into international treaties or unilaterally withdraw from organizations they are members of, then they are not sovereign.
e.g. The UK was able to leave the EU without the EU's consent, but the same cannot be said for Nebraska leaving the US.
Also, US territories are self governing. e.g. Puerto Rico and Guam. They aren't governed directly by the federal government. The only major difference between a territory and a state is in voting rights and congressional representation. I believe there's an exemption from federal income tax as well.
The theory is that the states forfeit certain aspects of their sovereignty, but not their entire sovereignty, by entering into a federation with the other states in the United States. The specific aspects of their sovereignty that they forfeit are those powers explicitly enumerated in the Constitution. As you note, this would include the ability to enter into treaties. But the 10th Amendment (at least in theory) guarantees that they retain their sovereignty in all other areas of governance.
There is still a distinction between US territories and states, because while in practice the territories are self governing, they have no Constitutional right to self governance. The federal government grants them some degree of autonomy as a matter of pragmatism, but the federal government has the power to revoke this autonomy by a simple act of Congress. The same cannot be said of states.
