1. Its way easier on me. I don't have to worry about password resets or users hacking an account in the site. OAuth and be done.
2. I don't have to deal with any outgoing or incoming emails and complaints dealing with login.
3. I don't have to worry about scammers, since google/facebook pretty much require phone, a non fake email, 2 Factor auth, etc to sign up. So I can put all that overhead onto google and get the added benefit that most of the users signing up are legit.
4. Legit users. I can't say enough how much having legit users helps the overall quality of almost any platform. Allowing people with a fake-email to sign up and create accounts allows for all kinds of headaches that are largely avoided by using exclusively social sign in.
5. Simplicity. One Click login, where the user immediately sees what information I want to collect and can say yes/no. Login/Sign-Up is done in literally one click.