That's what a diet is supposed to do. If they don't lose weight it is because they are not expending more energy that they are consuming. While people vary in the amount of energy they need per day (and therefore also vary in how easy it is to lose weight) the bottom line is always the same: at some point, if you keep eating less you will start to lose weight. 

That's what a diet is supposed to do. If they don't lose weight it is because they are not expending more energy that they are consuming. While people vary in the amount of energy they need per day (and therefore also vary in how easy it is to lose weight) the bottom line is always the same: at some point, if you keep eating less you will start to lose weight. 

Firstly, that isn't as easy as it sounds like, given you have the money and all the possibilities to buy a lot of different attractive food.

Secondly, if you just eat less of everything, you get not only less calories but also less vitamins, minerals and so on. That's what a good balanced diet is to compensate. An unbalanced diet (eating ONLY *** and %%% for ### months, claiming to help you get rid of harmful stuff accumulated in your body) might make you lose weight fast but it also makes you ill.

Because I sit here all day, posting answers and eating HobNobs.

Without disagreeing with the other replies, I suggest that you re-phrase the question in terms of sex, drugs, vanity and similar matters concerning excesses in personal gratification. You should find that answering those questions would deal with the "Why"?

It amounts to a matter of one's "will power" exceeding one's "won't power".

Remember also how Bierce defined "Debauchee": " One who has so earnestly pursued pleasure that he has had the misfortune to overtake it."