Because it’s not. Thermodynamics is a law, not a political or mental thing! My maximum effort in a day was an 8 hour off road bike ride. I burned 3500kCal ju

Because it’s not. Thermodynamics is a law, not a political or mental thing! My maximum effort in a day was an 8 hour off road bike ride. I burned 3500kCal just from the riding, as measured by a power meter. The body can’t even absorb enough carbs to replace that in the same time. There is some difference in efficiency between people for muscular exercise - between 20% and 25% of energy burned actually gets converted to work. And no people cannot extract more or less. They can absorb more or less in total for carbs - between 80 to 120 g/hr balanced between glucose and fructose is the limit for most people.