I dont understand why nobody plays full AP Soraka

Full AP mid is OP. Seriously I never see Soraka go full AP as a Mage. She is kinda rediculous. Max her E first, Rush and you basically have free trades E procs the slow making it an easy hit for Q that creates a bigger slow that traps enemies in the E when it ends causing ever more damage and binding them. late game she dominates teamfights creating widespread damage and crowd control all while have a rediculous heal. that makes her next to unkillable and a heal that brings carries up to full. I build and completely dominate. Sure her laning phase is weak and her aa is really awkward to use. once she gets she is an unstoppable force.