[SteveNuts]

The problem is it's much more complex to manage N users in N applications, having a central location to onboard and offboard your users is a huge boon to IT departments. The convenience isn't for you as the user.

[Spivak]

That's central identity management -- related to, but fundamentally orthogonal to sign in once be signed in everywhere.

SSO without central identify management would be weird but also very possible.