Banks are aware that NIST and various other bodies have updated their guidance about password expiration. Even vendors like Microsoft who supply extensively to financial services, have updated their guidance about password policies.
At this point — barring edge cases of operating in geographies where regulations haven’t caught up — it’s just inertia, aka “inaction doesn’t get you fired (usually)”.
I have been in three different organisations now with this same excuse, and actually called their insurer to clarify. In all cases, the insurer asks the password policy such as expirations. Complete absence of a written policy is a problem. Non expiring passwords was not.
Someone in management took the application form and justified their own belief on security and two of those three companies still tell staff "it's because of our insurerer" even after given the facts.
At this point — barring edge cases of operating in geographies where regulations haven’t caught up — it’s just inertia, aka “inaction doesn’t get you fired (usually)”.