Can I provide a counter example?
The ALSA asihpi driver is about 90% shared code with osx and windows drivers.
The private source is transformed by (scary) sed and python scripts into
the public version visible in the kernel.
Part of this process replaces OS-abstraction wrappers with their equivalent kernel direct calls. Datatypes and symbol names are also transformed to
kernel style. Other-OS-specific ifdef blocks are removed using a simplified preprocessor.
The result is not particularly pretty, but it allows the driver to exist by minimizing the linux-sepcific development and maintenance. It is much less likely that this driver would exist if the entire thing had to be rewritten and maintained separately.