Is English your first language? If it isn't, I completely agree with you.
However if it is, then I would make the argument that it's exactly the right way to teach it, particularly to young people. As you point out, it's a very fluid language with a lot of rules that are completely arbitrary. As a native speaker, it's more important getting to grips with the, I dunno what we'd call it on here, pseudocode rather than formulaic structure? Concepts over syntax maybe.
I'm from the UK. My immediate family is from Dundee and Sunderland. I had inlaws from Liverpool and Bristol. When we all got together, especially after a few drinks, at no point would an outsider think we were even speaking the same language, but we all had the same common grounding.
I agree with you that it's probably a bad way to teach formal grammar. It depends on the context though