You're thinking of the wrong definition of master. Git's master refers to the primary copy of something from which derivative copies are made. E.g. In casting, a master is used to form new molds[1]. In audio engineering, a master tape holds the final mix of a recording[2]. (Incidentally, this is where the term remastering comes from, because you are creating a new master version.)

1. https://www.instructables.com/id/Two-Part-Silicone-Casting/ 2. https://en.wikipedia.org/wiki/Mastering_(audio)