I'm trying to guess why, and can only imagine that at some point the idea was to support multi-window webapps, like maybe you'd have separate windows for documents and different tool palettes, and they'd want to know each other's locations to try to prevent overlaps? I mean, back in the day, framesets were another idea that experienced some popularity before being totally eclipsed by <iframes> -- all of this back in the era when everything was based on the multiple-windows paradigm rather than the multiple-tabs paradigm. Window.screenLeft apparently originated in IE [1].
Just a guess though. Very curious if there's any legitimate reason for it to continue to exist, or whether W3 ought to deprecate it.
At first I thought this is an electron only thing. I get it that we gave up on fingerprinting resistance but this is like pissing on it's grave.
EDIT: privacy.resistFingerprinting set to true fixes the coords to 0