It's Mentally Exhausting

Role playing is an exercise in mental fortitude. You are pretending to be a different person with their own life and philosophies, in a place you have never been, interacting with other characters, solving challenges, and doing all of it in real time. It's a significant expense of energy.

For as much as it may be taxing to players, though, it is exponentially more exhausting to the person running the game. The Game Master has to be mindful of every character in the game, has to actually control every character other than the players', and has to come up with explanations and descriptions for everything that happens in their world.

Imagine you are a writer and you spend 5 hours writing a story through the process of a group of people telling you what one person does, then you taking note of that and writing the results of those actions. Certainly it can be done, but by the end of it, you will be wiped out from the experience.

This is another reason I really respect role players as storytellers. They are doing one of the most difficult forms of storytelling out there: continuous, spontaneous storytelling that incorporates the input of others.