The scrolls are created by neither Aedra nor Daedra nor mortals. In fact, they are entirely otherworldly. Hence they can speak about events outside mortal comprehension and across the timescales of the universe. What an Elder Scroll is, is not something I'm very knowledgeable about, but I would assume that the Greybeards view them as blasphemous as they are means by which men see and know that which is even outwith the scope of the gods. Moreover, the Dragon God Of Time, something that the Greybeards are associated with, is both bound by the scrolls and, in his breaking, creates events which the scrolls cannot even talk about. Basically, think of the scrolls as a potential history of everything and everything that will be. This is not an adequate description, but it shows how they are in some way opposed to the one who shapes and maintains time. When the dragon breaks, then the scrolls are of no use. Indeed, this is a good question and I hope someone else can help uncover the mysteries.