Thats like saying running the car does little for gas burning because you can fill up in 5 mins.

You have to actually kick your ass when you exercise. Most people don’t push that hard. Michael phelps ate what 6k calories a day or something crazy to maintain his prime form with the amount of caloric burn he was doing in training. If he continued to eat like he did in those crazy olypmpian training years, he’d be morbidly obese. If he ate like he does now in those years, he’d starve.