I can't tell you why something is done without seeing the evidence that it is done. Apparently you have some reason to believe this is taught; and you may well be right. I need to see the evidence you have, and in particular just what is said and by whom. As it is, your "why" question is just a cover for what is merely an accusation (like asking "Why do you beat your wife?").
You've made a broad claim implying that all teachers in your area teach this as a matter of course; how do I know it isn't just one teacher who said this once, and was taken out of context? If you have no evidence for us to evaluate, then this thread should not exist.
It happens that I had a conversation just this morning with a fellow tutor about the fact that elementary teachers don't know what they need to know. If she had made a specific claim, she would have gladly given me specifics.