Game / System
As a game, I would love to say because of the fun of it, but it is simply not true. Come to think of it, D&D and other RPGs tend to eb rife with frustrations, coupled with very funny moments, but that's not the same thing as watchign LotR which was wall-to-wall fun, is it?
No, the main reason at my middle-age progression is the mild illusion, for a brief moment, that we actually CAN make a difference.
System wise, no other game has as much material written for it or so many variants. No other reason. What most people seek out of a pen & paper RPG can not be done and will eventually become a DM'd form of computer game, but we haven't the technology as yet.
I also love Runequest...