Music theory is the theory behind music the same way grammar is the theory behind sentence construction in natural languages. It is descriptive - it is not nece

Music theory is the theory behind music the same way grammar is the theory behind sentence construction in natural languages. It is descriptive - it is not necessarily searching from some universal "laws of nature" and nor is it axiomatic like math. It is just studying and describing the structure of music. Some parts of theory seem to be universal, some are genre specific. Music notation is used for communication among musicians and is optimized for clarity in expressing real-world music. Real world music is based around scales (except for 12-tone music which is a really obscure niche), so notation is based around scales. (Although different genres may use different notations, like chord notation instead of notes.) The underlying relationships of wavelength ratios is interesting part of theory but you don't really need to know about that to play music. But you do need to know about notes and scales, so this is what the notation is based on.