Come on, you've got a password manager that saves passwords and usernames. It couldn't be more convenient to login.
Why would you give the responsibility to google for your logins?
Why would you lock yourself into the vendor google by using their login system for every other service? You can't migrate anywhere easily.
I'm just not enabling such a method. It's not implemented. People who don't think about it and hence don't care usually still use the service eveb if they cannot use "login with google"