Most people who talk about music theory are artists, not scientists. Scientific rigour isn't normally needed or even expected by people who create music.
What really happens in my observation of what you describe: musicians learn about what other people claim as theory and then pass it on without much question. However, the "theory" itself doesn't just come from practicing musicians explaining their own work (though there's some of that). It comes from a history of theorizing.
A good analogy is language. Poets may indeed chatter about grammar a bunch. But the grammar they describe is usually stuff they inherited from their own schooling and from grammar books that have a whole long multi-generation history. And regardless of all this and the fact that it's largely right overall, scientific study of linguistics has found all sorts of issues with traditional grammar ideas.
Anyone interested in understanding language, whether theoretically as an end in itself or to explore the art of poetry, will do better by studying scientific understandings of linguistics than in memorizing old grammar textbook claims.
Language is an interesting topic. Words ultimately mean whatever it is that they communicate. The meaning of music theory is what musicians have established it to be.
For what you are talking about, you probably need to be more specific. Maybe scientific theory of music.
I'm suggesting that even to musicians, when they say, "I want to understand how music works", they pick up a book on "music theory". They think it does mean that. And they study it and hope it will get them what they want. It does a little. They aren't presented with the idea that any other approach even exists. And when they get advanced enough, some of them teach "music theory" classes and write more "music theory" books — all still thinking this is as good as we have at explaining the nature of music because they themselves sought that out and this is what they got.
At grad-school level "music theory" all the insights from music cognition are recognized as totally within "music theory" except that everyone is so invested in the general notation-jargon path to get there that they rarely think much about what theory could/should be for beginners. They eventually found the other stuff after so many years, and most people don't question whether there might be a better path.
