They had decided the best course of action was in the open field and try to get the skeletons to gather and face the Lich head on, they didn't want skeletons falling out of the trees, and crawling up some pass. And since the area was a wasteland for a long time there shouldn't be too many bones in the ground that would be usable.
They had a formation of their troops mixed when the griffins were about to begin. The skeletons are really just the bones, so the flesh rots, they would still fly with recently deceased animals with wings, but it was limited and eventually their feathers would fall off.
