Why getpid() is not implemented in x86_64's vdso?

Question

After glibc 2.25, glibc's getpid() wrapper no longer cache its result. However, on x86_64 vdso didn't provide getpid() function. Which means everytime getpid() is called, a syscall is triggered.

I am wondering why x86_64 vdso does not provide getpid() function (even on kernel 5.x).