I think the key to the rationale is in the name. libc is a library for C. Go doesn't use the same calling conventions as platform C and does a number of things that make it not really play well with C code.

The authors want a world where you can just make a read syscall without using FFI.

It's possible this is a unrealistic expectation. Linux kernel interfaces are generally quite stable. On OSX and windows the kernel interface seems to be an undocumented unsupported API which is sad.