>>76415168Because
1 - It's not possible or easy to quantify how many calories you spend with different forms of mental exercise.
With physical exercise, it's somewhat doable to estimate how much you burn per hour by sprinting, swimming, walking, etc.
How would you even measure how many calories you burn by doing math vs. learning a language vs. learning how to play an instrument?
2 - mental effort is far more painful than physical effort.
People would much rather run or walk on a treadmill, accomplishing nothing productive, than use their brain developing a skill.
3 - People do exercise for other reasons than just burn calories.
They may want to develop muscles, their vascular system, develop strength or endurance, etc.