Bungie might prefer to expand the universe through books or other media. But I think the Mass Effect codex entries expanded the universe without too much effort, i.e. just write about something, rather than have to imply it through dialogue, location, environment, etc. Bungie might think that's too easy though, maybe they prefer a subtle touch. For example, you could probably play through the entire Halo series and not have much regard for the universe as a whole. But Bungie slip in little bits that imply past events, like in ODST, when Buck shouts out "It's like Reach all over again !". So the player (who has read none of the books/ other fiction) will know that something significant has happened outside of the scope of the game they're playing. [b]TL;DR:[/b] A codex might not be subtle enough for Bungie to expand their universe, they might prefer a lighter touch.