Because forked kernels is how we ended up with all those SBCs which could only run some ancient version of linux. If you can't mainline it, you are committing to maintaining a fork forever, and the more changes you require, the more maintenance that fork will be.

The Linux kernel quite intentionally makes maintaining a fork a pain in the ass to pressure vendors to mainline their drives. But now mainline maintainers are refusing to accept those drivers. So it's unlikely we will ever see Macbooks properly supported on Linux.