That's the thing - authentication capability is basically sideeffect of Oauth/OIDC. SAAS x can request whatever they want via OIDC and then user can either accept it or decline it. This is protocol, not google specific matter. Ask ordinary user how they understand what is being asked from them when they are trying "to log in" via OIDC/Oauth. With SAML it's the other way around - administrator chooses what is being sent to SAAS x, user does not need to decide anything nor do they get hard to understand prompts.