No demos is just the way gamesas seems to roll. From what I've gathered, they're squeamish a bit about "incomplete" materials and designs possibly giving expectations of the final product, when they prefer to leave as many options open for them to change right up to the game going gold. This is ALSO why we've tended to get (often frustratingly) little information about games up until the release date: fewer promises means fewer promises that could be broken.
As far as a demo goes, it would actually be relatively easy to do one: just get a stable compiled build (the hard part) and pair it with a set of "demo content." The problem for gamesas, of course, is that "demo content;" they don't want to be pinned down with whatever they show. The closest thing, that MIGHT work, is if they instead decide to make a demo out of intentionally scrapped areas: ones that they'd note would not be appearing in the final game.
They could do that, but the demo would be 8 gigabytes. Then someone would find a way to hack it and give you the full game.
Code != content. The code (excepting the shaders) for
Oblivion is stored entirely in Oblivion.EXE: only a few megabytes. It's surprisingly compact once you compile the stuff into binary.
The content itself can be readily adjusted, IF one has the time to do it. Mods have been made for MW/OB that outright replace the main quest and land, placing the character in what is effectively in ENTIRELY different game. The only question is gamesas's willingness to put the time and exposure into assembling some of this content into a demo. Another issue is, of course, the fact that the "great outdoors" is PART of the whole game experience... And hard to capture in a restricted demo. You'd need to make a lot of detailed scenery with nothing in it just to make sure there's places to look at when outside, or not have it at all.