Because all of this is regulated by hunger. If you move more, you get hungrier. The issue in the case of most overweight people is that their body is trying to

Because all of this is regulated by hunger. If you move more, you get hungrier. The issue in the case of most overweight people is that their body is trying to maintain a set point at a high level of body fat. Hunger serves to maintain this weight set point. The way to sustainable weight loss is to eat in such a way that the body starts to target a lower set point and decreases hunger. Obviously this also results in less calories taken than expended, but if it were as simple as a math formula most people would have six packs.