Well, the game doesn't take easily to new animations. That, and even adding missing ones is a time-consuming process(as in months), which most aren't willing to devote. Not because they don't want to, but because anyone who mod games doesn't want to do it all the time.
Its almost the same reason that new body models are close to impossible, since not only would the model have to be made, it would have to be given animations. Though most of the existing models pull their animations from a "supermodel"(not in the fashion show sense), I don't think you could just make a body model and link it to one of them. Could be wrong about that, but I'm fairly sure that is the case.