Would you still want it if it meant the game would only be half it's size?
That's basically what any sort of multiplayer would do - double the amount of code needed for the same thing. And that's not counting support for people joining / leaving a game already in progress.
And then there's the whole gameplay style, say waiting or resting - what happens to the other people in the game? What about fast-travel? Would you simply teleport from place to place, without time passing?
And since the world levels with you, if a level 20 starts a game, and a level 10 joins, would he get killed by every monster he'd meet? Or would the level 20 suddenly meet much weaker enemies than before the other guy joined?
Generally speaking, any sort of multiplayer would mean a much smaller gameworld, and a much simpler one at that.
There are solutions to all of those. They have been addressed in countless games for countless years. I'm also reading a lot of assumptions by people, in regards to give and take. "If we have co-op we will have to give up content!!!" Do you really know this to be fact? I don't think you are qualified to make such a statement as that. Look what the modding community for TES can do. I'm playing a heavily modded version of Morrowind right now, and it feels like a completely different game than the one that came out in 2002.
What brings me back to elder scrolls years after a game's release is not vanilla content but everything the modding community does for this series.
What takes me away? The boredom of playing alone.
I do not care what Bethesda will hold out on if they release some form of co-op in future TES, because I am not concerned knowing what the community can do with the game.
You are worried about synchronizing waiting and resting in multiplayer? I can not help but laugh when I see your signature includes a nod to Neverwinter Nights.
There are plenty of good reasons to adapt to the ever growing market of multiplayer gaming. There are also plenty of bad reasons not to. And I am reading a lot of them here.