Why do people feel the need to eat at the movie theater?

If you want to eat, why can't you eat BEFORE you see the movie? It makes no sense to me. I was watching "The SImpsons Movie" and this girl next to me was gobbling away as if she hadn't had a decent meal in years -- hot dog, popcorn, nachos, soda, the whole works. I mean seriously, who started this tradition of eating at the movie theater and why?