Because, unfortunately, the majority of people nowadays don't teach their children basic manners. The amount of times I've seen a child biting, kicking, having a tantrum, swearing etc. and the parents have just sat by and done nothing or justified it with "kids will be kids". I may only be 21, but I was never like that as a child and if I were to ever misbehave in such a manner my Nan would make sure I never did it again!
One memorable occasion was when I was on work experience, and one little boy of 6 attacked a boy of 5. It was entirely unprovoked and completely vicious. He even picked up a chair and hit the poor boy with it. When I asked him why he told me what his father had told him. Which was if the child didn't like anyone, or thought they were beneath him, he could do whatever he liked and if they didn't like it they could shut the f**k up.
I was agog. And then when the father turned up and basically told me exactly the same thing - that I was not to discipline his son for "doing what comes naturally" - I almost cried with disbelief. Absolutely shocking.
And yes, that's true. However I'm sure I'll get the sceptics whining about how it couldn't possibly be accurate and how I'm just making it all up for effect. 
But anyway - people seem to not want the responsibility of teaching their children in the proper manner, either because it's too difficult or because they are instilling the wrong type of value. And when those children grow up, they'll do exactly the same to their children. It's a vicious circle but it's not one we can do much about.