None of my stories center around the eight main characters in the game, but that's probably because I started writing before the game was released and we had any hard details about them.

These days, yes most stories are about or relate to someone's character in game. Some follow the progression of the storylines, some ignore it. Some expand upon the times before, some focus on the times after. Some revise certain things to suit their narrative needs, others just fill in the little holes. Further developing the relationships between the 'heroes' and their companions is probably the most popular style/genre/focus.

I find that slightly odd, it seems like a topic that would get mildly stale after a little while.

Since most writers have 'their own take' or 'their own interpretation' on events transpired that already have multiple outcomes in game, it provides a wide enough variety of options for people. Then again, different writers with different styles can make the presentation of the same event itself seem vastly different. And much of the Short-Fic writings just involves various topics and ideas expressed through the 'lens' of certain characters, not being direct tellings of events we see in game.

I find that slightly odd, it seems like a topic that would get mildly stale after a little while.

In my experience, not really. Sure, the same basic premises and NPCs are present, but everyone's interpretations of events are different. The individual player characters also tend to play vastly different roles within a given storyline depending on their own experiences and personalities, which makes for stories that are always fresh. Besides, people don't always follow the game strictly word for word. There is room for interpretation.

Besides, people don't always follow the game strictly word for word. There is room for interpretation.

And some of us merely exist in the setting, rather ignorant of game events unless those are significant moments, such as there being a war going on.