Perhaps the reason is instinct. Primitive man either ran after his food, or away from animals to which he was food. Our immediate forebears also had their huge physical tasks, just for survival. A navy moving ten tons a day. A miner 12 hours at the coal face.
Evolution takes tens of thousands of years.
I was running away from a sabre-toothed tiger. The following is the rest of the piece which I didn't send to the magazine. I woke up in the middle of the night, in my bed at home. Wider awake than normal, as if alerted by a sudden noise. I assessed my position in bed, I don't know why.