Because it’s their software? It is well within your bank’s rights to deny you access to their online banking system for pretty much any (technical) reason they choose; why are you entitled to run their app on what they deem to be an insecure platform? If you don’t like it, either pick a different bank or deal with not having access to their software.

Freedom cuts both ways here; if you want absolute freedom to do whatever you want with your device, why should software vendors not have absolute freedom to choose what platforms their software is permitted to run on?

Obviously none of this applies to FOSS.