> Other reason is that kernel requires some minimal level of code quality and many of the closed source ones are basically "the minimum to make the customer use case work"

I think this is the real, actual reason. Having worked in embedded for a long time, the coding horrors are endless and routine. There's no way 99.99% of embedded software would ever pass the kernel's strict code quality standards. It's easier to just ship a "bastardized" (as others here are calling it) patched kernel.