It isn't laziness in the teaching part of manners, IMHO. It's laziness in correcting children when they violate the rules of manners and politeness. Most parents don't want to be bothered. I mean, all you have to do is look at how much true quality time parents typically spend with even very young children to see how they just let stuff like that slide.

Many parents also feel it is a personal afront when someone else corrects their children's behavior. I think it's pretty clear that's why other adults tend to not correct children when they misbehave or even to bring it to the attention of the parent: they don't want a confrontation or to be yelled at for simply being the messenger.