Why do you want me to log in?
I want to show you how well you have been doing playing against the bot. You need to have a user ID, so that I can keep score of your results and show you your overall winnings.
Why can I only log in via Google / Facebook / Twitter?
I am not interested in your personal information. Relying on social ID providers is both the easiest way for me to implement user authentication and the safest option for your privacy. You don't need to share your email address with me and you don't have to rely on me keeping your password safe. The social ID providers only share a minimal amount of information (basically a user ID name) with me.