Mining is precision work, not just mindless swinging of a pick. You have to aim every hit, judge swing angle for every hit, adjust power for every hit, watch what is happening to the rock/soil you are hitting for every hit, and be aware of constant hazards. It is both taxing on the body and mind. I have never mined, but I have cut trees with an axe, which is very similar to mining in what is required of the body and mind. I get tired both physically and mentally.
So, there you have your answer. Anyone who has done hard, precision work already knows this. Common sense.