I'm vaguely aware of what single sign on is. Though I don't use it, it seemed non-intrusive. The most you would see buttons to login through Google, Facebook etc.
Now some sites have an entire banner pestering you to sign in through your Google account. It has a big continue button that takes you to Google and the button to close is a small x in the corner. On mobile this banner takes up a third of the screen.
Why do this?
No, they don't get paid. Its just an easy way to integrate your app with some sort of login/authentication floe, rather than building your own.
There are SSO solutions besides google. For example, Keycloak is open source but it is a PAIN IN THE ASS to manage it and set it up properly. Most people take the easy route and I don't blame them.
What I don't get why have this weird ugly banner for advertising Google's SSO. I often see it on Reddit's new interface for example.