They want to stop people from using tampered or fake versions of their app which could be tricking the user into doing things. They also want to enforce using a device with a mandatory app sandbox and some level of security patches/protections intact. Most aren't willing to ban using highly insecure Android devices which haven't received security patches for many years, but it's trending in that direction.
Around 90% of banking apps are currently compatible with GrapheneOS. It would likely be more like 50% if we hadn't put a lot of effort into improving compatibility with a lot of workarounds for the bugs and problematic anti-tampering checks done by these apps. It would be closer to 20% if we not only didn't have that but also didn't have our sandboxed Google Play compatibility layer feature.
The remaining apps not compatible with GrapheneOS is mainly due the Play Integrity API device and strong integrity levels enforcing having a Google certified device running the unmodified stock OS. Google doesn't allow GrapheneOS to be certified. Certification also disallows many of our privacy and security features. It also isn't compatible with providing fast security updates and would waste substantial resources as it does for Android OEMs. Those apps need to be convinced to stop using the Play Integrity API or to use the Android hardware attestation API provided as part of key attestation to verify the hardware, OS and app are genuine. The latter only solves it for operating systems and hardware they choose to permit.