Heh, Neverwinter Nights had swishing capes with mods.
If they already have to do skirts, I'd be surprised if they didn't have swishing capes (unless they make the skirts follow an animation pattern, in which case perhaps we can prescribe Bethesda something to take for their physics-phobia).
So did Oblivion. And Morrowind, for that matter, although they didn't swish in that one, so I never used them. The Oblivion ones are animated similarly to the hair animation technique I described earlier, so they at least follow the character's movements.
It's not physics-phobia, it's just laziness. Every single item in Oblivion (except, for some reason, paintbrushes) is modelled using Havok. Now this does absolutely nothing to enhance the gameplay, if anything it makes it worse because after every fight you have to comb the place to find where the @#$% that guy's enchanted sword went. So the only reason for it is that it's eye candy. Now a solid object rolling around is impressive, but naturally flowing cloth and hair is much more impressive still. But it's also harder to do. Based on the fact that Bethsoft shoehorned physics in even at the cost of making the game more annoying, I'm absolutely sure they would've loved to have that. But it was just too hard, so they didn't bother.