I recall a happy/fun environment using Microsoft Entra (Azure AD) SSO, in order to sign into Okta SSO, in order to access Azure environment(s), among other apps. SSO Inception.

SSO chaining is super common in large corporate environments. Different orgs might have their own SSO IDP, acquisitions often bring their own, etc. Once a provider is in use, it is quite difficult to tear out later while keeping everyone in their proper accounts in all the apps that tie in. Many apps are really bad at SSO migrations, or deduplicating multiple SSO identities to a single user account.