Remember OpenID? Yes, that's what it was for, OAuth wasn't never meant for signing in other websites who just want your mail or something... Of course, all these big tech corps quickly dropped OpenID, they don't want people to control their online credentials or identity...
Has there been any retrospectives or published thoughts around why OpenID failed? Ideally a extensive, impartial report would be nice to read through. While it's easy to blame big technology companies for the failure of open standards, there might be other reasons behind it (as well as companies trying to prevent it from succeeding)
Why would anyone bother with that hassle when you can just put in your email address (that you already have & know) and a password.
In contrast, OAuth succeeded because most people already have a Facebook / Gmail / Github account, which meant that sign up just becomes clicking a single button which is easier than email signup.
OpenID was more difficult than email signup, whereas OAuth is easier.