I see Conway's Law at work here. The marketing department must have its own IT department separate from the IT that maintains the core website and business functions. It's impossible for them to get on the same web domain (much less build something in the phone apps). Instead, they built their own disparate site and experience.
It gets worse. These German "Sparkassen" are small to at most medium sized credit unions. They are organised in a larger umbrella organisation that takes care of some of the services like IT, but the individual banks can pick and choose what and how much they want to handle themselves.
Some of them are larger and pretty well organised, but there are also a lot of small ones that just don't have the people and expertise for things like proper IT security practices. But customers trust them, because they position themselves as these local neighbourhood banks, even though most of them are pretty incompetent and will rip you off with high fees on accounts and shitty, underperforming investment products.
