*nix is a weird contraction because it doesn't (lexically) include Linux, even though everyone reads it as if it does.

Or FreeBSD or AIX or ...

You might feel better viewing it as a way to avoid the trademark on "Unix" than as a contraction.

Historically, to me it comes from trademark. During my time "Unix" trademark was owned by AT&T Bell, USL, and then Novell. They really did not like it when others used "Unix".

Lot of things do not make sense when we look back historically. It did make sense back then. We still have many such thing lingering around.