Even Go gave up and switched to using libc for OpenBSD[1]. You can use syscalls on Linux because Linux, lacking a blessed libc implementation, guarantees stability of syscalls as an interface. The BSDs, afaik, do not, so circumventing libc is liable to render your binary forwards-incompatible with future kernel versions.
That isn't just about the lack of guaranteed stability, as far as I understand it is considered a security feature. System calls that are not made through the libc may be actively blocked. So an attacker can't just use an exploit to inject a system call into a process, the call has to pass through the libc wrapper and in combination with ASLR that wont be trivial.
A better way of accomplishing this would have been to randomize the syscall numbers on a per-process basis, and map a "syscall number translation table" into the process when loading the executable.
[1] https://go.dev/doc/go1.16#openbsd