The problem is convenience
Sure, it is very convenient to Sign in With FaceBook/Google/Microsoft.
On the other hand, security experts have been constantly repeating for years that you should not use the same passwords for all sites you sign up for.
How does that compute ? It doesn't. Where does that get us ? To this sort of problem.
I never sign up with any 3rd-party identifier. I manage my own passwords and I don't sign up to social platforms (well, Google signed me up for Hangouts when I got my Gmail account, but I'll be damned if I use it).
I'm glad they found solutions to correct the issue, but I still won't use those kinds of services.
Good luck hijacking my 24-character passwords.