Most of the states didn't voluntarily join anything. They were allowed to enter the Union, admitted only after a majority of the existing states gave their approval through a vote in both houses of Congress. Shouldn't leaving have the same requirements as entering?
No, and you know why, because the subject has been beaten to death on other threads (and you lost).
Once a State, a State is a State is a State. Nihil Obstat if a State wishes to assume full sovereignty.
Some States, it has also been pointed out to you, were States before they were members of the Union, and that includes States that were not members of the original Thirteen Colonies. I refer to, and we have discussed at length, the cases of Vermont, Texas, and Hawaii.
California "could" be thrown into the mix, but their "Bear Flag Republic" was a put-up job, like the Lecompton Constitution in Kansas (I'm sure you will agree about that last). So arguendo I leave them out.
Any people anywhere, being inclined and having the power, have the right to rise up, and shake off the existing government, and form a new one that suits them better. This is a most valuable,-- most sacred right--a right, which we hope and believe, is to liberate the world. Nor is this right confined to cases in which the whole people of an existing government, may choose to exercise it. Any portion of such people that can, may revolutionize, and make their own, of so much of the teritory as they inhabit.