I guess people don't do it because the difference is minimal and they use a lot of other features from the C runtime too.
EDIT: Ops. Not on BSD! The entire thread is about BSD and here I am mindlessly talking about Linux.
[0] I seem to remember that this changed in a recent Windows version, but I couldn't immediately find a source.
[1] msyscall(2) or, if you don't have an OpenBSD system at hand, https://man.openbsd.org/msyscall