Is there some reason that the Linux kernel couldn't offer a stable API for device drivers? At least for common types of devices. Would that really lead to ossification? Stable APIs usually make it easier to change code at lower levels.
Politics. Offering a stable-ish internal API would instantly lead to hardware vendors shipping closed source drivers for Linux.
Currently, you have to be the size of NVIDIA or AMD to be able to afford a closed source driver, it's simply a huge work to keep up with the constant improvements in the Linux kernel.
Embedded/mobile projects don't tend to use bleeding-edge kernels and stuck on one version regardless of closed source modules anyway.
[1] https://arstechnica.com/information-technology/2020/03/the-e...