@GrapheneOS the system is open source, what stop you to implement it and even better contributing to it to improve security?
Because this system is a very good idea to reinsure the banking company and European Union and it create a viable alternative to the Play Integrity.
Your approach to just say the security rely on the user didn't convince any big firm as they are legally still responsible in case of issue and the law on that is still protecting consumers.
The responsibility will remains on the apps for consumer protection so we need an alternative to make it that way and Graphene OS is not providing anything for that.