Well, I think that children are often the mirrors of society - if they see nothing good worth following, they follow what is bad.
I know for sure that lots of the elderly people are first and foremost rude themselves - but when they see the exact same behaviour they are showing on us, it suddenly becomes an outrage, lack of manners on our side. Parents now often do not have the time or can't be bothered to spend the time with their cchildren. And then they wonder what the heck had grown up from their children.
Noone seems to be bothered by the fact that sometimes, it's not entirely the child's fault they have no manners, as they had no person to learn from.