Comment by skydhash

Because at first your have this

  main <- PR A <- PR B

Then you'll have

  main, squashed A
      \
       \-> PR A -> PR B

The tip of B is the list of changes of both A and B, while the tip of main is now the squashed version of the changes of A. Unless a branch tracks the end of A in the PR B, It looks like more you want to apply A and B on top of A again.

A quick analogy to math

  main is X
  A is 3
  B is 5

Before you have X + 3 + 5 which was equivalent to X + 8, but then when you squash A on on X, it looks like (X + 3) + (3 + 5) from `main`'s point of view, while from B, it should be X + (3 + 5). So you need to rebase B to remove its 3 so that it can be (X + 3) + 5.

Branches only store the commits at the top. The rest is found using the parent metadata in each commits (a linked list. Squashing A does not remove its commits. It creates a new one, and the tip of `main` as its parent and set the new commit as the tip of `main`. But the list of commits in B still refer to the old tip of `main` as their ancestor and still includes the old commits of A. Which is why you can't merge the PR because it would have applies the commits of A twice.