Many banks I have worked with require between 4-6 characters passwords. Some even force to use only digits.
Such a poor password is protecting your financial information, because MFA is usually not available. It's truth that when you want to perform an operation, you get a confirmation either by SMS or the mobile app, but to check information this is not required.
I'm quite curious, because banks which have implemented a lot of security features like BBVA, still use such passwords. So, probably there is a reasoning behind this, as it's most banks, not just one or two.
I refuse to believe it's due to legacy systems, because as I said, if they have implemented other security features along the way, I hardly doubt they could not deal with 20+ years old password requirements.