What incentive would a bank have to release their apps as FOSS?
You probably could create an open source banking app and use it to run a bank on a primarily open source software stack. But banks are not software companies, and they have no reason to engage with the FOSS world. We could think up lots of potential reasons for why a bank might not want to release their apps as FOSS, but the simplest answer is "why would they?"
I'd love to live in a world where free software is the norm, but we're not in that world. So if the bank has no incentive to do it other than the comparatively niche interests of the FOSS community, they just won't do it.