Second, it often still takes a lot of work to create a new account on a site, even with a password manager. Selecting a username, discovering it's taken, selecting another one, generating a random password, pasting it into a second field to confirm the password, unchecking "send me updates", going to my email to find the confirm link, blah blah blah.
If I just want to do something quick on a site (like see a Quora answer or Medium post), it can be far easier to just click "log in with Google" and see the content in 5 seconds rather than 5 minutes while you wait for the damned account confirmation email.