It's not a silly question. Based on the experiences that were made with GearVR, it turned out that strapping a phone to your headset before starting to play inc

It's not a silly question. Based on the experiences that were made with GearVR, it turned out that strapping a phone to your headset before starting to play increases the barrier and makes it significantly less convenient. And, convenience is king. Furthermore, there are a lot of different smartphones out there with different performance attributes. This makes is much harder to make a compelling VR experience, because you trend towards the lowest common denominator. Another point is that while yes, a smartphone has almost the same components as a VR headset, you can optimize the components and space much better if you are using it for a specific use case (which is as a VR headset) and not a general purpose phone. Finally, it's much easier to keep your VR headset charged and ready to go compared to relying on your phone to jump into VR.