I've said it before and I'll say it again: "Video Games are art." Like all forms of art, not every single game (or book or movie) is a great example of it, but as a medium, video games deserve respect as an art form.
In no other way can someone experience a story the way one can in a game. They are interactive experiences, and nothing else can hold a candle to how involved in a story a well-made game can make you feel, or force you to ask yourself questions because you're the one making the decision.
That being said, sometimes works are transferred from one medium to another with varying degrees of success. The Lord of the Rings were books that got made into movies. There are only six Star Wars movies (right now, anyway) but the Expanded Universe fleshes out that world with hundreds of books.
Obviously, a story owes much of its power to the medium in which it's told. Movies utilize lighting, music, and images to make the audience feel like they're there in the action. Books allow the audience access to the innermost lives and feelings of main characters. Video games give the player the ability to control how the story is told. So what happens when a video game story is adapted into a book?
Really, I can only think of one example I've read personally, and that's Halo:Combat Evolved. Essentially a strict novelization of the events of the video game, reading it was like watching someone play the game in my head. That's neither a criticism nor a selling point, just a fact.
This whole blog post comes from the fact that I am a novelist that plays video games. When I got to the end of Bioshock 2, I was disappointed the story was over. "How neat would it be," I wondered, "to wander the halls of Rapture, to see it through the eyes of one of its inhabitants, to make an attempt to see it as a real place, rather than just a set piece which my character moved through?"
The same goes with Portal 2. How thrilled would I be to get a call from Valve asking me to create a novelization of their game? The answer: Super thrilled. I love these games because I love the worlds they allow me to inhabit for a little while. Writing a novel about them would grant me the chance to live in that world for much longer.
And true, I could write fanfiction. But I get antsy about writing things I know I cannot sell. Writing is writing, sure, but I want to be a professional writer, I want to make a living off the words I put to paper (or more realistically--to screen).
So I wait, and hope, and dream. Maybe one day I'll be a big enough name that somebody will want me to create the novel tie-in for their property. Meanwhile, I'm waiting for that email, Valve. Any day now...