Pensionista is right to ask about how children come to be so bad mannered. We do have to teach them as little children, which we did, back in the early 1980s. But my friend back then was a Brown Owl at a local Brownie pack, and taught the little ones manners because they were not being taught by their parents. Their mothers were openly grateful, and not one of them showed any shame for not having taught them at home.
My two lads joined the Australian Army Reserve while at university. (The money was a big help to them) Their Dad, an ex-Able Seaman in the RN in England told them they couldn't join unless they went as officers. He assumed our working class background would prevent this, as it could have done back in our day in the UK: he didn't really want them in the armed forces. But they both got in, and went through Duntroon (our Sandhurst) together successfully, and were immensely grateful for all that training in manners. Some cadets had to be taught how to eat properly at the table! And I've read about firms sending their aspiring executives on etiquette and manners training courses!
It is just like the correct use of English: they can't do it unless they've been taught. There are so very many parents out there who have no idea about how to bring up children, and the sad truth is that most of them just don't give a damn.