The kernel isn't working on commercial deadlines. Maintainers don't have to worry about their stack rank at the end of the year. They don't have to justify their head count. Some of this happens internally I'm sure, at places like IBM, Red Hat, and Intel, but none of it is coming from Linus.
If someone was to design a good small set of opcodes we could go back to rolling machine code by hand. While at it we might as well fill the atx box with cute tiny isolated computers that talk over ip addresses. They have to be small enough as to not leave space for an OS. Some ROM is fine ofc.