States did not have the power to enter. They were allowed to join only if the other states agreed to let them in. So if the power to admit states was a power delegated to the United States then wouldn't the power to let them leave also be delegated to the United States as well?
The power to leave is not explicitly delegated to the federal government, therefore it is retained by the states, according to the bill of rights.
How can you read the ninth and tenth amendments any other way? In light of Patrick Henry’s involvement in crafting the Bill of Rights and his authorship of the ratification agreements of New York and Virginia, which explicitly retain the right to withdraw from the USA, it is a slam dunk. These States would not have ratified with these guarantees in the BOR, and there would have been no ratified Constitution.
PS - Did you know the original articles confederation were written as perpetual, but this feature was removed from the Constitution?