[soundwave106]

Probably the correct way to phrase this is Western sheet notation was designed around the diatonic scale. Western art music rarely deviated a lot from diatonic until the late 19th / early 20th century.

Another way would be to say, "most music historically has been built out of 7-note scales", because from my perspective, that would be correct... the vast majority of music systems in the world are either heptatonic or pentatonic. There are obviously exceptions (Gamelan scales for a start -- https://en.wikipedia.org/wiki/Slendro), but to be honest I can't think of any historical octatonic or higher scales at the moment. I'm sure they exist, but they seem pretty rare (until the late 19th / early 20th century classical era that is).