You just answered your own question.
Because it wont work on the Xbox or PS3, it wont be done.
Multi-platform releases are all well and good, but all parties need equal treatment or you end up alienating a third of your customers. HD pack worked for the PC, because it wasn't a HUGE deal, and PC fans were less likely to stone Todd Howard in the streets after they got it.
To release a potentially game changing feature like Spell creation to PC only would show preferential treatment to one platform... which would be akin to shooting one's self in the foot. Not to mention totally unfair, and BS because it would lead to different features available on different platforms, meaning multiple releases of the same game, meaning multiple copies one would need buy to get the whole experience, then someone would say that they are discriminating, then it would result in bad press, then Extra Credit will do a video about it, then we'll hear about it for the next 5 years as the biggest failing in gaming... etc.
For them to do it now would be a BAD idea.
Just wait. Modders will figure it out, or it'll be in TES 6, one or the other. We'll all get Spell making eventually.
Makes sense, I guess. But if it was something of an "official mod" like Howard was saying some of this stuff might be, and released for free, I doubt anyone would care.
I bet if they did release this stuff only on PC though, thousands of Xbox players would google "how do I ride dragon"

(edit: I don't mean anything by that. I have Skyrim on the Xbox only so far, as my PC is bad. Someday I'll be getting it on PC as well - same thing happened with Morrowind and Oblivion.)