If 'why are the curtains blue' were consistently explained together with Chekhov's gun, then maybe we wouldn't be here having this discussion.
The foundation of the mystery novel.
Being able to perform critical analysis of text is an essential skill today. It might be more essential now than any other moment in history. Understanding how narrative writing uses symbols translates cleanly into understanding how political messaging or any persuasive writing uses symbols.
You can just teach the thing you want to teach.
1. Novels considered “curriculum-approved literature” often have symbolism that is irrelevant to a student’s life. It was placed there intentionally by the author, and was blatant to all readers when it was published, but it is indistinguishable to a student from the teacher making things up.
2. Teachers who aren't the best end up teaching from a “it's true because it's true” mindset, which may as well be “because I made it up and said so.” These are quite common.
3. Or the teacher draws from a pool of stock symbolic and thematic answers for all novels. Astute students will spot that immediately and treat it as a game of guessing the teacher’s answer rather than engaging with the text.