I have disabled PAM by default on all my boxes that run sshd for the last 9 years out of habit, I long ago forgot the reason why (probably because the gentoo sshd handbook entry said it was a good idea). Why UsePAM is set to yes in sshd_config by default on many distros is beyond me.
Because PAM is the default authentication framework on all those distros. Yes, it's a disaster of complexity and something pretty much no one understands. But it's what we have.
Maybe a rearchitected replacement will land in systemd someday...