Consider that kids develop habits in childhood that will carry into adulthood. It benefits the kid to understand the basic rules of politeness. Otherwise, they might wonder why someone else got that promotion, even though they work just as hard. Or why that one co-worker won’t talk to them. Or why that girl/guy they were interested in suddenly isn’t interested in them anymore

Being polite is a way of signaling that you are willing to follow certain social rules. This is important so that communication can happen. If I follow different rules than you do, it could result in conflict, confusion and misunderstanding