Because the body can only extract so much energy per minute from all of the fat in your body. If that's not enough, muscle is used, etc.
Was curious about this, went hunting for some rough data, this [0] suggests every kilogram of fat held can be drawn down at ~70 food-calories per day.
So someone with 25% body fat weighting 100kg (~220lb) could draw 1750 food calories per day, which strikes me as pretty ample unless they're also adding a bunch of physical activity.
It seems likely we've evolved to reduce energy expenditure in other ways when we regularly induce physical activity, too. Walk 20,000 steps or spend a couple of hours on the treadmill? Your body finds ways to reduce your energy expenditure elsewhere.