This has the nefarious side effect of allowing Google or Facebook to track people across the Internet and apps. Webmasters like you are, often for no imperative reason, complicit of this by providing such login options.
App developers have repeatedly stated that offering those options increases user account creation. There is lower friction to using “login with <big tech>” than to create username/password creation flow. My guess is that most of the world hasn’t figured out a password manager workflow that works for them (or they aren’t willing to pay for it).
I don’t know what the exact revenue/growth difference is, but if my paycheque depended upon getting more users to sign up, I don’t think I could justify making it into a political stance when Google isn’t going to notice my tiny boycott.