Probably because eating something shitty can be done in like 2 minutes.  "I wanna be fit!" they scream, but 23 hrs a day of wanting isn't enough if you break down for 15 minutes 4 times and eat 2000 kcal each time.  10 minutes is less than what it takes to go thru a drive thru and down whatever calorie bomb they send out.    I'm not saying it's not their fault, I'm just saying if you're habit is to eat a ton of food and you decide to get fit, it has to be an all-in kinda thing.