Pragmatic reasons:

I use chown moderately often. Usually as root, I want to change the uid of the files after generating a bunch of files. But I almost never changed gid.

I am in two mind whether I should add lchown as well or not (just like lchmod) and make the signature of pathlib.chown same as os.chown (with third parameter, follow_symbolic_links).

If you close this as won't fix, I will not hold grudge against you, though. ;)