And that's because it copied heavily from NeXTSTEP of the late 1980's.
See https://en.wikipedia.org/wiki/NeXTSTEP#/media/File:NeXTSTEP_...
That beveled look was invented by Steve Jobs's team, and copied by Microsoft.
I would much rather use NeXTSTEP look & feel than Windows 10 or even OS X.