For me,
the answer was really nicely put by Eero Tuovinen:
I find that the riddle of roleplaying is answered thusly: it is more fun to play a roleplaying game than write a novel because the game by the virtue of its system allows you to take on a variety of roles that are inherently more entertaining than that of pure authorship.
When I was young and got into RPGing, I enjoyed comics and novels. I don't read many comics these days, and fewer novels than I would like to. But I do watch more movies, more seriously, now than I did back then.
As well as experiencing stories of characters doing things and making choices, I enjoy creating them. But as Tuovinen says, it is more fun to play a RPG than to write, because of the entertainment that RPGing permits. I like inventing situations, or - in the case of Prince Valiant - presenting situations that Greg Stafford or someone else invented, seeing what the players have their PCs do, responding to that. Sometimes I laugh with them. Or compliment them. Often, especially in failure-and-suffering-laden Torchbearer, I make fun of them (in a friendly way). Occasionally I'm amazed by them - here's an example from TB2e play:
It doesn't quite come through in the write-up: but after I described Megloss blasting Gerda with fire, the chatter at the table was light-hearted and bantering. I can't remember what we were talking about, but I noticed that Golin's player was not participating. And as I looked to see why not, I saw he was building a big pile of dice. "What for?", I asked. "Because Megloss killed my friend. Here are my Fighter dice, and my weapon die, and my Avenging Grudges Nature dice on top of those." The calmness yet intensity of the delivery, the determination - there was not a hint of hesitation in his description of what he was doing - were striking.
Golin's anger towards Megloss for his murder of Gerda - the complete opposite of
recovery - and his determination to avenge her death, were palpable. The huge pile of dice visually conveyed it. At the table, this moment felt as powerful as anything I can remember from a Claremont X-Man comic. It was blood operatic.
I want these moments. Not because
I've authored them, as GM, but because play yields them through its own dynamics. So I choose systems that can reliably produce them.