I feel like people who ask this question believe co op is a magical spell one just casts on a game.
For one, would you enjoy playing co op and getting disconnected evey half hour due to freezes? That's just one simple reason.
But hey, I understand the want for co op. Just, think about it for a few minutes and you'll realize it's just not feasible.
BTW No one is stopping you from creating a DnD campaign based on Skyrim. Sounds kind of fun IMO. --- http://www.youtube.com/watch?v=GtxCr5ju2EE&feature=related - If you played Mystical Ninja 64, you need to click this.