In Transformers Prime, Megatron twice attempted to Cyberform the earth (once in Darkest Hour and once in DeadLock). He did this in the belief that by cyberforming the earth, he would wipe out both humanity and the band of Autobots there.
However, as we learned in One shall rise, in this continuity the earth is actually Unicron's slumbering form. The last time Unicron was waking up the earth was bursting forth with Dark Energon and the enemy of all that lives was able to create an arguably infinite number of deadly stone avatars. They only managed to disable Unicron with the matrix of leadership, but as the movie shows, his anti-spark still exists: he is very much not dead. Therefore, wouldn't it have been a foolish mistake to risk waking up Unicron?