If you do a whole bunch of exercise and create that calorie deficit, that means you can eat some more, right, so long as you do enough of said exercise? Unfortunately, it's not that simple.
You can hit the treadmill and burn off 500 calories and then eat crisps with a caloric value of 500 - that seems like a fair trade....but it isn't.
Junk foods lack nutrients that your body needs, like lean protein and wholesome carbs. If your body can't use the food you put in to repair muscles, etc, it will simply be stored as fat.