Earlier quoted context omitted.
This exists in some European countries, in Hungary for example you have an identity service (KAU) which authenticates you and operates as an SSO provider across a number of different government properties. The United States has it, too: https://login.gov But with a government as large as America's it's going to take time to get everyone converted to the new system.
FWIW, as a regular user of login.gov, from the outside, it looks like a well-designed system. I am able to add strong forms of 2FA (e.g., security keys or biometric authenticators), it requires strong passwords, etc. It also has decent developer documentation, has a support process, and comes with a vulnerability disclosure form baked into the main website. However, I have not used their API, nor have I seen any of t…
I believe most of it is open source: https://github.com/18F/identity-idp