Because the numbers back it up. People flock to Facebook (1 account for many things), Google (1 account for many things), Amazon (1 account for many things), Apple (1 account for many things), Reddit (1 account for many things). Blogs are dying, forums are moribund, small online stores are dying.
HNers discussing about Facebook/Google logins in their apps regularly mention that more than half their users use those options, instead of creating an account using their email.
Just go ask a bunch of your non-techie acquaintances if they use those web browser features you mention.
Sorry, I don't think I communicated my point properly. I agree with you that the average user doesn't want to deal with separate credentials. What I mean to say is: why is this the case?
If a signup page auto-fills with your email, name, and suggested secure password, and then your browser stores and syncs that information, that's basically as frictionless as clicking a "Sign In With Google" button.
HNers discussing about Facebook/Google logins in their apps regularly mention that more than half their users use those options, instead of creating an account using their email.
Just go ask a bunch of your non-techie acquaintances if they use those web browser features you mention.