Because Performance Based Navigation (PBN) assumes you can fly more accurately if you are in GPS Primary than tracking a radial from a single navaid. It uses an already approved procedure but with far more accuracy. The navaid if not used, is no different from any waypoint on a RNAV approach.
A note of caution - by definition any conventional fix is the equivalent of a flyover waypoint, and the obstacle protection areas are designed accordingly. RNAV approach waypoints use a flyby attribute as a default, and the underlying obstacle protection area will not allow for this.