Part of the problem might be that grammar is often not taught in high school English classes. During my career as an English teacher, I was often the only person in my department who either knew grammar or was willing to teach it. And yes, I had a privileged education at a Canadian university that is often compared to American Ivy League universities. I found that my grammar lessons seemed to enormously helpful to my students if they were going on to university, especially if their intended studies involved essay writing.
Creative writing does not require strict adherence to standard English grammar and formal usage. Teachers can have students rewrite dialogue from a play in a regional dialect, for example. Or students can write a memoir in their own voice, whatever that might be. I did all of these things.
There are ways to get past this seeming dichotomy between standard English and academic writing: I think teachers can assign different kinds of writing for different purposes.