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.
Tangent: I signed up for a US TD account recently (in person). They had me write down the username I wanted, so I used LastPass on my phone to generate another random username. They obligingly made me an account with username "ajdgsbrjcobsdhfwvfk" - and password "tdbank123". Yes, I was required to change it on first login, but no, there was no attempt to verify that I was the one doing the changing (birthdate, SIN, etc).