You have it backward. The conventions were discovered first, music theory came after composers did things. Music's theory describes what we already know works, not how to make things work.

No composer I know thinks in the terms of music theory. They often use a chord progression that music theory describes why it works, but they're not thinking of it in terms of music theory. They're thinking in terms of chord progression, just like the composers of thousands years ago were thinking of their chord progressions the exact style changes and so a thousand years ago they went to thought in terms of modern chord progressions but they were still thinking in similar terms.