It takes superhuman willpower to go to bed hungry every day when you have the option not to. That's why people on low-calorie diets so frequently (read: almost always) put non-trivial weight back on within a few years.

Your brain tells you to eat n calories. Your brain will make damn sure that you eat every bit of n. Telling a person who can't eat less than n that they're lazy is like telling a schizophrenic that they should just snap out of it.

Will is an exhaustible resource, most people don't have enough to overcome the desires of the body. It's not as simple as calories in calories out; it never has been.