Mostly because primitive flutes and horns historically had those seven notes (plus octave) [1], or they sounded good when sung or played in sequence or together

Mostly because primitive flutes and horns historically had those seven notes (plus octave) [1], or they sounded good when sung or played in sequence or together. It "just made sense" to stumble upon them, because of the harmonic series [2]. Some intervals were more obvious (octave, perfect fifth, major third). Others probably took hundreds or thousands of years to be discovered. But keep in mind that different cultures used different scales. It wasn't always the same. Some had minor seventh, others major seventh. Mesopotamians used a sharp fourth. This is still visible in different cultures today (blues/rock uses a lot of flat seventh!). As for "why seven?": Since "mixing" major/minor sevenths/thirds/etc is very dissonant and weird, people ended up having seven notes regardless. I would say that "note choice" was more cultural, but the options were obviously influenced by the harmonic series. After a while people started seeing patterns and those historical scales converged into the major scale we know today. Later in the 1500s some geniuses found a way to transpose the scales but still maintain the ratio between notes, but without having to retune the instrument. The trick was to divide the octave in 12 notes but only use seven at a time [3]. It wasn't "perfect" like just intonation [4], but it was in the ballpark. That became the new normal. Equal temperament is not perfectly in tune with the harmonic series, but people got used to it (to the point that just intonation sounds "off" to a lot of musicians). [1] https://en.wikipedia.org/wiki/Music_of_Mesopotamia https://en.wikipedia.org/wiki/Music_of_Mesopotamia [2] https://en.wikipedia.org/wiki/Harmonic_series_(music) https://en.wikipedia.org/wiki/Harmonic_series_(music) [3] https://en.wikipedia.org/wiki/Equal_temperament https://en.wikipedia.org/wiki/Equal_temperament [4] https://en.wikipedia.org/wiki/Just_intonation https://en.wikipedia.org/wiki/Just_intonation