There are a few reasons Universal hasn’t done a pure “video game” house. One, the big one, is that video games are a “limited market” compared to movies. While they’re popular, their fan-bases narrow to those who have played the game. As popular as Bioshock is, more people have seen Us. Then, there’s the complication of getting an IP approved by management.
Let’s say A&D is proposing KKfOS and Bioshock to someone who has not seen or played either for approval. The approver can go home, take two hours, and just watch KKfOS. They could watch a full play through (8-12 hours) assuming they can even stomach watching someone else play an FPS (I rarely can). Or, they can play it themselves, assuming they have a console for it, but that is still a much bigger time commitment than a movie. And sure, A&D can put together a sizzle reel, but that doesn’t address potential questions the approver may have.
That’s the reason, I believe, Silent Hill and Resident Evil are the only video game movies they’ve done; they both have movies for the approvers to watch to get a full sense of the IP. Similarly with a TV show. A&D can say, “watch these 2-4 episodes.”