Maybe I'm missing something. Nearly every one of these companies offers SSO on their basic plans through the big public IdPs (Google, GitHub, Microsoft, etc).

What's being advertised as a feature is not SSO, but SSO through a private IdP. So this could just be a case of confusion created through marketing simplification.

Which, fair game! If you are big or technical enough to need a private IdP, you should probably be paying for an Enterprise plan. And from the perspective of these software companies, supporting your third-party IdP is kinda a luxury feature. Moreover, I can understand why Adobe wouldn't want to include these advanced features in their plans for college students.