I think it's the parents responsibilities to keep reminding children. Unfortunately many parents don't follow-thru with polite manner teaching to their kids. I'll keep trying. I think it's combination of things. I have a lots of my friend from all different cultures complaining from the same things.
Some people say that girls are better than boys.
I don't think so. Maybe they are shy or they think is not cool to be seen talking to their friends parents . I keep telling mine that they did not say hello or thanks or whatever that is appropriate loud enough. So keep reminding them. They will get it right sooner or later.