Mobile devices are often limited by memory bandwidth, and by OpenGL driver overhead. Consoles have very little driver overhead, so number of draw calls is not as big a problem, but state changes are still costly, so doubling the number of state changes hurts performance. I could imagine some clever techniques to avoid doubling the number of state changes (perhaps a geometry shader that duplicates triangles to render both eyes at once) but I don't know how well that would work. VR rendering is still mostly unexplored!