It drives me absolutley insane. I understand that some cultures do it as a sign of satisfaction. But why do so many Americans do this (especially students at my highschool)? Ive noticed alot of people dont even relize they do it or even completely deny it. Its either a full on SMACK-SMACK-SMACK, or a few subtle smacks when they are about to swallow, followed by an odd whining noise. Whats even worse is when they make moaning sounds, growl, and breathe heavily when doing so. Is it supposed to taste better that way?
Bad parenting is the reason. If the parents don't teach their children to have decent manners, they will most likely end up savages. Occasionally, people will raise themselves out of the gutter and learn on their own, but most don't. So they will probably stay savages. Unfortunately, most parents don't take their parenting responsibilities seriously enough, and just want schools to raise their children for them, even though, obviously, a school is no substitute for parenting. If they did take their duties and responsibilities seriously enough, they would probably decide to not have children, as doing a good job of raising children is an incredible amount of work. Most people quite simply are not up to the challenge of doing it well.
Since you are talking about manners in an American school, customs in other parts of the world are irrelevant. In any case, I doubt that most of your classmates would be models of politeness in other cultures, either.