For context, this is how it worked in old-school UNIX. If you look at (for example) SunOS, the kernel ABI is undocumented and everything in userspace depends on

For context, this is how it worked in old-school UNIX. If you look at (for example) SunOS, the kernel ABI is undocumented and everything in userspace depends on libc. The OSes and userspace languages of the day were deeply coupled -- C was the language of UNIX. You can still see the remnants of this design in macOS, except instead of libc it's libSystem. Apple has decoupled their OS from C (in favor of Objective C, then Swift) but the idea of a userspace entry point to system functionality survived. Linux is different from traditional UNIX because it's a kernel-only OS. There's no "Linux libc", and third parties have written multiple implementations of the C standard library that run on Linux. The only way this structure can work is for the kernel itself to provide stable ABI, and the standard approach at the time (mid 90s) was numeric syscall codes with positional parameters.