Why do people insist on chewing with their mouths open and/or smacking their lips when they eat?

It used to be frowned upon as a "lack of breeding" but essentially what it means is a lack of basic training form childhood. If you visited their homes you'd find their parents exhibit exactly the same disgusting behaviour. Monkey see, monkey do. Simple. The saddest part of course is that they don't know they're doing it & so it falls to someone else brave enough to tell them, otherwise they will pass it on to their children and so it goes ad infinitum (forever)

One of the reasons why i hate going to the movies, all that chomping is horrible.

Ugh i hate that. and when people smack their gum and take it out and play with it !

ugh, that's disgusting, it's a habit I guess, I remind them everytime, I don't wanna see what's going on in their mouth

Bad manners, I guess. Its disgusting, and it really bothers me when adults who should know better do it.

they were not taught the right way when they were young.

they stupid