Maybe you wanted to embed the login page on your own site (iframe or webview)

Half of the reason to use SSO is that users should not ever enter their credentials on arbitrary web sites.

Otherwise generally agree with the rest of your points. Acquiring a token from a web page and especially authentication from non browser contexts can be excruciatingly difficult. Imagine an idp only providing login through browser prompts, when you are trying to build a CLI that requires login.