Because it's a single player game. Not every game needs to be designed with multiplayer in mind, y'know.
Matt, it really comes down to choice, it doesn't have to interfere with how you play the game if you don't connect to Xbox Live/PSN/Steam with a friend of yours.
Ha, no reason to apologize, I believe that's why I worded it that way.
And I'm going to completely agree with That. A lot of quests in Elder Scrolls games are really not two person friendly. They'd have to completely recreate the entire setup. You can't just have quests to go somewhere and kill someone. It's only good for multiplayer if it's Go somewhere and kill Everyone.
Yes, there'd be a bit of working, however, as someone pointed out, do what Borderlands did, I can't help but agree. Difficulty scales with how many people are in your group-session.
(I reminisce back to the annoying difficulty level of Baldurs Gate 2 in LAN with my brother, with the game being at max difficulty.)