Earlier quoted context omitted.
Exercise is a bad way to increase calories out, because it's just too efficient. The conversion is something like 1 mile run = 1 tbsp peanut butter.
And people almost always overestimate how much they've burnt, and often eat more to compensate.
After a bit of training, muscles can store carbon-hydrates for about 1.5 hours of exercise. When these carbs a burned, muscles take new carbs directly from the blood stream.
So if you exercise by burning mostly carbs and don't eat, you end up with low blood sugar levels and a very strong urge to eat.
The trick is to burn fat. But to burn fat in a straight forward way you first have deplete the carb store, and basically force muscles to burn fat. This requires exercise that lasts many hours in one go.
The additional benefit as far as I know is that if muscles are trained to burn fat, they will start burning more fat even when not in use. So the effect lasts a lot longer than just the actual amount of exercise.
So we have a culture where exercise means to go to the gym and do exciting things there. Which mainly burns carbs.
In the past when people did manual labor, they worked the same way for an entire day, burning mostly fat.