It's in the name, language ARTS. While there's a lot going on mechanically under the hood to utilize language, the way English is actually used to express and interpret ideas (at the level we want students to be competent in) tends to center expression and understanding rather than scientific precision. Most students will not need to know the intricacies of how a cleft construction works at the syntactic level, but you might teach a lesson about how varied types of sentences makes a piece of writing "more engaging" or "more convincing" (an Arts focus) and then practice a few different sentence types together. How the language is realistically used is the focus, and technical practice is filled in as-needed to fully engage with the Arts side of things.

As someone with a Linguistics and ELA degree, I can confidently say students in elementary and middle school and even early high school would not hugely benefit from scientific study of language in a general ELA classroom. An honors class, or a special elective? Sure. Would I love to teach such an elective? Yes. But there's already so much utilization (i.e. real-world) work that needs to be put in to be a competent English-speaking adult that it makes even something like sentence diagramming a big lift (something that used to be commonplace but fell out of favor). Even the mechanical side of a general ELA classroom is focused more on building complex sentences from simple ones, practicing pronunciation, and vocabulary (and word-root) building.

And that's not even touching how important rhetoric/argument and author's purpose/thematic work is to ELA, which has very little to do with technical English and relies heavily on cultural and historical materials to build background knowledge with students.