I would also mention that gpus are a GIANT abstraction, and since they are rev'd faster than arguably any hardware in a sytem, there are abstractions layered on top of that for the families and models of gpus too.

Another way of looking at it is -- I started playing with openwrt for a relatively small router, with 5 ports plus wifi.

I was amazed at not only the amount of openwrt code required to support the different router families and the different router models, but at the sheer amount of stuff turned on by default in the kernel just in case I might need to load a module for some obscure feature or package. I assume the same goes for a gpu driver both at the source level and in the kernel.