It's almost identical to saying "just stop taking drugs." Or "just stop drinking."

The reasons people turn to drugs and alcohol are not entirely different from the reasons people turn to food, but you have to keep eating something, and changing your diet from a very unhealthy one to a healthy one is a lot of work. You can keep going to the drive through, but a, they're literally designed to get you to buy more than you want, and b, would you tell an alcoholic to go in to a liquor store for soda on day 1 of recovery?