You don't have to rewrite the source branch to squash merge?

I wouldn't describe it as "cleaning up the history". And the goal isn't to save space, it's to keep a linear history where things ought to be working at each commit (to enable tools like git bisect and similar).

I personally don't ensure everything is working every time I commit - That's what CI is for. The exact process I work through while writing a PR shouldn't impact other people's workflows, so when I merge back into a central branch it should really only reveal the granularity at which I assert 'this code is working and good', which is NOT every intermediate commit I make. Squash merge is a way to do that that fits nicely with existing engineering workflows, like code review.