Some of them are published authors, some are game designers, and some are just good writers. Fans have written game-related fiction since there were games to write about. My first piece was a diary of a Runequest adventure sometime in the early 1980s. These were written for my own amusement, and later for increasingly high-quality gaming publications such as Mindless Slaughter, PBM Scroll and Games Without Frontiers. The web offers new opportunities for publishing fan fiction of all kinds, and game fiction in particular.
Any idiot can stick a game-related story up on a website.
For one, FFN only publishes fiction with the permission of the game copyright holder. Fiction is cleared with the designers and fits the official game milieu. Secondly, FFN only publishes quality fiction. Most is written by published authors and game designers. Our fiction is literate, faithful to the setting, and fun.