[eklitzke]

How else would you implement PAM? The whole point is that you can write arbitrary code to implement whatever custom authentication policy you have, so it needs to be able to dlopen things. And if you don't like the PAM policy written by your distro, you're free to change it by implementing your own authentication policies, or dropping things you don't care about (like systemd, you can configure PAM to just use /etc/passwd and /etc/shadow if you like).

sshd needs to run as root (obviously) because it grants login shells to people, so it needs to run in a privileged context. And the PAM modules it executes also need to be run as root, because PAM modules need to do things like read /etc/shadow.

[JoachimSchipper]

https://man.bsd.lv/login.conf.5#AUTHENTICATION documents OpenBSD's system, which revolves around running /usr/libexec/auth/login_<foo>. OpenBSD's system doesn't let you do the stack-of-libraries thing that PAM does, but having one binary is a lot simpler.

(In the past, OpenBSD had a login-locally-or-via-Kerberos binary there, which does show the downside of that approach over PAM's more flexible approach.)