It's just a security measure. As a founder of a currently 4-person company, I want SSO everywhere I possibly can. It reduces attack vectors, and makes it so much easier to ensure nobody has access when they leave the company. Every product we use that doesn't offer SSO has to be added to our onboarding/offboarding checklists.

It comes down to this: Don't assume companies are incompetent at proper dealings around employee access to products they use just because they're small. These things tend to be correlated, but it hurts small companies trying to deal with this correctly.

Edit - Let me phrase it like this: By locking away account management and security tools you're implicitly stating only large enterprises should care about security.