There's no reason that can't be done in a TES game, of course. The problem with the MMO, however - and keep in mind I've never played one, I'm just going off others' descriptions - is that it seems waaaaaay too much like work. Everything just sounds like grinding, grinding, grinding. That and cartoony graphics. WoW always looked like crap to me, certainly compared with the visual awesomeness that is Skyrim.
Some can be, especially your Korean style MMOs. But in some ways, I would say that they're not necessarily more grindy than Skyrim. If you remove or ignore the whole "tell your own story" aspect of Skyrim, what are you left with? A game where you run around, kill a bunch of stuff, fetch random items, rinse and repeat. I have a tendency to drop characters around lvl 40 in Skyrim. Why? Because by that time I've done most of what is appropriate for that character and I don't have much more of a story to tell about that character. MMOs provide you with many other people to play with, stretching out the possibilities to be nearly endless.
I played WoW for more than four years on an RP server. I stopped playing several years ago, but I had characters that I still think very fondly of. Not because of what they accomplished in the world, like raiding this dungeon or getting that loot drop, but because of their personalities and interactions with their fellow guild members. Whether that guild was a military organization set up to defend the Alliance from Horde attacks or a school intent on training healers to better aide the Horde. Sure, it may be cartoony, but it's also what? Seven or eight years old? And it's cartooniness has helped it age better than a more realistic game would have.
Then just this last weekend I participated in the Guild Wars 2 Beta Weekend Event 2. Here someone is taking an MMO and incorporating a strong personal story for each player, creating a world that is dynamic in ways that we could only dream Skyrim was, and filled it with mechanics to encourage spontaneous grouping and cooperation instead of griefing and competition. It was only one weekend so far, but my impression was that it was quite the brilliant game. And after spending a weekend with it's UI, coming back to Skyrim's UI (even with SkyUI), was downright painful.
Anyways, that is getting a bit off topic. At the very least it pertains more to TES:O than to Skyrim, since Skyrim will never have anything close to those kinds of experiences.