That's very strange. Here in Norway we have a unified login system for banks and state services that lets you set up the login with your own bank then any other service that needs you to log in will delegate the login process to your bank and never needs any of your details except for your personnummer (social security number in US/UK I suppose). Then your bank either asks for a code from a one time pad or sends a message to the sim card on your mobile and asks you to confirm with a pin code. It then tells the other entity that you are who you say you are and you are connected. It's called BankID. Beats me why practically everywhere else is so primitive.
Yeah, I heard from a colleague (so anecdotal) that apparently it's because Germany in general is quite behind in terms of digital services, which in part is due to still very patchy broadband in large parts of the country.
Yep, same in Denmark. NemID. But it works with any bank, public service etc. The govt runs the IDP and any company or public institution that wants to utilize trusted user login will delegate authentification to the govt idp