Because if he already had them, there'd be no need for him to go to the past and get them. Or in other words: if he already had them in the future, he would never be able to get them. That's not very consistent, is it? Unlike the Song of Storms, this wouldn't be a self-fullfilling, but a self-denying 'prophecy'.
Hope that clears it up since I didn't want to discuss this further. I understand that the split timeline is handled very much like a dogma around here, so there's not much sense in trying to explain my points. Most of you would probably just consider me stubborn and narrow-minded for doing that, so I'll stop.