Because the OEMs and carriers see them as integrated products and not as systems running an open kernel and open windowing environment. They don't see any point in separating the two, defining a clear interface specification between them, and upgrading the system software to match the hardware. If they did that they could release an update to the userland system that matches the kernel-land and lower level radio and hardware interfaces. Every android device could run an upstream-derived (and tracking) kernel and core system layer, with the binary graphics drivers and other closed components released on a schedule. Android, as delivered by Google for the Nexus series could run on every device.