Lack of centralization is one part of it (see also: communal digital services), yes, but the complete lack of standards and guidelines is also a massive issue. I tried buying a Deutschlandticket from the DB Navigator app a while back, and immediately ran into some issues:

- they only take credit card, probably because of the massive SEPA fraud they've had happen

- they require id verification with a third party(!), which then only supports the e-perso(!!) or video ident(!!!), which they could've just used the actual PostIdent service for, which would've provided an alternative for non-smartphone-havers / people who'd rather not have their ID and face recorded by some Eastern European company until the end of time

- their entire authentication system was down when it came to actually purchasing

buying from my local Verkehrsverbund was a single tap in their app instead, with no id verification whatsoever. If DB's offering were the only option it would be an absolute travesty.