I wrote a big long post of my own on a similar subject sometime this summer, asking myself why I've only ever really gotten fannish about RPGs, and I came to a similar conclusion: games have hours and hours of content, and usually reams of backstory and lore to explore, but they also leave a ton of gaps to fill, both for the world and the characters. Much more so than your average book or film or even TV series. Exploring those cracks and discovering what might lie inside is one of my favorite things about writing meta and fic.