I'm not sure poorly defined is the best way to phrase it, more that things like the Miller Test recognize that the best way to define these things is that if a bunch of random people all find something obscene then it is, and that's basically what a jury is. Not ideal, but you can hardly rigorously define the concept and the whole idea essentially revolves around an assumption that if something is offensive enough to the vast overwhelming majority then they can ban it unless someone can make a very good case that it serves a meaningful purpose. If that's the working assumption then 12 out of 12 random people saying 'this is obscene' after contemplation and defense is a pretty logical method of deciding. Legal bizarre but we do have plenty of other examples where we use this sort of method, pretty much every law that includes 'reasonable' is basically punting to the jury.