For example how did blues players learn how to do the music they mastered? No school would ever dream of teaching it. So the great blues cats all had to learn on their own, figuring out harmonies, styles of voice leading, all of what the music did - without the help of textbooks. It is striking how many of the greatest blues and jazz players valued old 78 rpm records that had been cut by a great player - they were like treasure because they contained the real musical principles that people were trying to understand. So that was an example of a very different kind of "theory" but just as meaningful.
You are right, of course. I was referring to art-music in my post, but a lot that applies to other musics as well. These blues artists, the ones with great ears, did arrive at some of the same theoretical conventions as well. But not one individual did. They were learning from each other, exchanging ideas with one another, slowly reaching a consensus similar to that of the art-composers of the distant past, and it took a lot of time for their conventions to become 'rules'.
The blues artists created something similar without a textbook, but the art-music composer could not do this as art-music is incomparably more complex than blues and even jazz music (not that jazz cannot get highly sophisticated). Huge ensembles and complex harmonies that often modulate a lot faster, and do so chromatically, instead of mostly in parallel motion.
In addition, art-music's legacy and veneration lies on the written page. This I have tried to explain to jazz aficionados as one of the great differences between the two kinds of music. In blues and jazz the performance is much more important than the piece itself (hence all the improvisations or singing styles where the real magic in the genres happen). They are music of the moment (I don't mean that derogatorily). In art-music, no performance by any conductor will ever be greater than the work itself as it lies, skeletally, on the page; the bare instructions of its potential realisation. It is like a Platonic ideal. Rigid in its formal and notational content (no matter what some conductors used to do 100 years ago as to the latter). So, since it is the score that is more important than any single performance, orthography and correctness became equally important. It is one reason that composers and publishers go to the expense of publishing revisions.