Google and by extension banks, are claiming that the phone on, Android 9, without security updates AT ALL since 2009 is perfectly safe and secure to use.
Meanwhile, really well locked OS, hardened so well some of the improvements were later picked up upstream (both by Google and Apple), running _the_ latest AOSP version and releasing new security updates within hours is not considered safe and secure, despite assuring full chain of trust (including locked bootloader, verified boot, etc).
This is what Play Integrity does.
Of course Android supports better scheme, hardware attestation, but od course Google enforces their iron grip on the ecosystem, and instead uses the outdated, flaved system that certifies only the devices with preinstalled Google services running in the privileged mode. Snooping on everything you do and have.
Thats the reason.