Why do GM's complain about chess draining their system?

My son almost plays chess everyday for about 4-5 hours (weekdays!), and hes in top form everyday (unless he hits a slump for a couple of days up to a week, which happens every couple of months). And i am confused, why do people (especially GMs) take it slow on games, saying it drains your mental health, when my son, can play 5 hours a day and still be in good form throughout the week, while GM's run out of mental health a lot quicker?

 Because 1. your sons livelihood doesn't depend on it and 2.  your son doesn't put as much thought into positions as gm's do and 3.  I am guessing your son is somewhat young.  The older you get the harder things get.  I used to be able to play pool for 12 hours straight without getting tired.  Now I get a couple of hours and my blood sugar crashes and I lose all focus

Now put your son in to tournaments with equal or higher rated people and get him to stare at a computer for 2 hours memorizing openings and looking over opponent's games and analyzing their style and technique and mistakes and learning dozens of lines coming out of the Sicilian and understanding themes within the lines and practicing making plans in the middlegames and defending with tactics and reading articles with the newest ideas in them and trying to gain initiative through sacrifacing and preparing for a possible novelty in the tournament while worrying about time control and losing and trying to reach a drawn position so you can walk out of there with at least half a point and not disappointing people and getting enough sleep so you can play a blitz match and trying to guess your next opponents opening and...you get the point.

Can't believe this is all one sentence! lol  Forget about GMs getting tired, I was tired by the end of this.

A