Rebase creates an entire new branch and drops the previous one, it is a form of history destruction.

Commits are actually snapshots of the entire repository, not just diffs, so even if the diff is the same, if the base is different, it is not the same commit. And when you rebase, all the old commits will stay there until you run the garbage collector, and only if they don't have a head.

Eh, squash has its place, too. Sometimes I made a simple typo, so I make another commit that fixes the typo and I squash the last two commits.