The whole point is to use Unity's single high level device independent VR SDK (which enables many internal rendering optimizations), and to avoid plugging in yet another VR SDK in addition to a bunch of other separate GearVR, Oculus, Sony, etc SDKs, and having to write complex branching code (and build configuration scripts) in my app to support multiple APIs with my own ad-hoc layers of abstraction (which is costly to maintain and soon to become obsolete). That's Unity's job, and why I gladly paid them for a license.
Unity has a device independent VR API that supports Oculus, GearVR, Sony and other devices, but as far as I know it does not yet currently support Google's Cardboard SDK. They've promised to support it, but they haven't delivered it yet, nor said when to expect it.
Unless Google wants to pay me for wasting my time, the last thing in the world I want to do is to spend a lot of time and effort duct taping Google's Cardboard SDK onto the side of my existing app that already uses Unity's built-in VR SDK, and then Unity finally releases their the built-in support for Google Cardboard that they've been promising.
The potential and promise is there, but until I can just tick the "Virtual Reality Supported" checkbox in the Unity player settings to transparently support Google Cardboard, GearVR, Oculus, Sony, Vive, etc, the rubber hasn't hit the road.
Until then, you have to do short term dead end hacks like this, which will probably break every time anything it depends on releases a new version (which is often): https://github.com/ludo6577/VrMultiplatform