The problem is the release cadence, especially around mobile devices. Driver packages for them tend to be worked on right up to shipping, because they are developed in parallel with the hardware.
Android using the absolutely most head or tip version of the Linux kernel sounds like a QA nightmare of its own.
Mobile SOC has to have everything to start up the phone, as there is no bios like system that the driver is kind work through. Maybe this is a problem that could be solved, but it hasn't been yet.
The problem, especially around mobile devices, is that the vendors only care about a single-time sale. They just fork a specific kernel release, take a chainsaw to it until it stops crashing on their device, and try to never touch the source again. Very few hardware vendors put money into having maintainable drivers, because the next gadget they ship might have different chips in it, and the chips come from a subcontractor anyway.
In PC land linux driver support is not always day one as they are working on the drivers up to and sometimes after the release. Somehow the mobile vendors aren't capable of this.
OEMs would rather sell new devices that do updates for free.
The same happens in PC land with laptops, you seldom get drivers from Microsoft for laptop specific components, those come from the OEM, and get you what you get.
