because the Constitution did not create the United States. It merely constituted a government for it.
The document which created the United States of America, and where you will find the articles dealing with membership in it is the Articles of Confederation.
And no, they were never repealed or abrogated.
If they were, you'd find an article in the Constitution creating and naming the union - but you don't, because there was no need, the union already existed and it's framework is governed by the AoC.
Even as late as Lincoln this was understood and acknowledged. Only modern Americans are clueless about it because we aren't taught it.
Lincoln did not point to any part of the Constitution to argue against secession - he pointed to a (mis-interpreted) phrase in the Articles of Confederation.