That's often not that simple.
For example, SAMSUNG might build your device, and get all of their own code openly.
But now for the US model, due to CDMA, they need to use a Qualcomm processor.
That needs a blob, and Qualcomm won't release that under an open license, nor update it.
So the OEM can either just not have CDMA support, or accept foreign blobs.
It works like this all way down the stack, down to even camera chips.
And then these devices all have custom hardware. Often hundreds of customly designed parts, with custom drivers, only ever for a single device.
Think of the Moto Z Play, withthe replacable components. Samsung phones with facial scanners. LG phones with 3D display.
One-off features that'd never get mainlined.