because we use so much energy just existing (1200+ kcalories just maintaining body temperature for example) that the extra we burn up through moderate exercise is almost insignificant. Couple that with the exaggerated sense of entitlement most people get from a little exercise an a lot of people who take up exersice gain weight (i.e. I can eat this 1000 kcalorie piece of chocolate cake because I went for a 20 minute walk which really only burned up 200 additional calories over watching TV). It is really instructive to get a computer (wrist computer, pedometer, bike computer, treadmill computer) that estimates calories burned based on weight and time or distance. Then you will see how little food a little exercise equates to (walk half an hour, eat 1/3rd of a cookie to break even - of course better yet walk 1/2 hour and eat no cookie). On the other hand, nothing beats exercising and dieting combined.