I think the reason is because glibc is a fundamental OS component that always works, and if not you have bigger problems, you don't want to rely on other daemon

I think the reason is because glibc is a fundamental OS component that always works, and if not you have bigger problems, you don't want to rely on other daemons or libraries to authenticate the user. Using a daemon is against UNIX philosophy, and also poses a lot of problems, that daemon is just another piece of software that could fail for many reasons, and when it fails what you do ? You can't login to fix the problem, even as root and even in single user mode! So you are basically screwed. How many times you had a problem regarding user authentication on Linux ? I never had, I was always able to log in into my system to repair it even when I completely broke it.