as others have pointed out, it's not a tech issue, it's a support cost.

I have developed several SSO in-house. The various providers don't always align. e.g. sometimes you get the email, sometimes you get others, sometimes you don't get anything. It's a mess. If any of those details goes wrong, your user will be locked outside of your system and vent.

And there's another kind of fake-SSO. After the first time you login with SSO, the system asks for yourr phone or email again! So what's the point of SSO?