All bank sites I know have a pin like system that locks your account after 3 failed password attempts. This means that long passwords would be counterproductive because they are easier to mistype or forget, especially because you can't see them (and if you could see them security would be even worse).

If you take eg. a pin system for smartphones where you type in 4 numbers, the chances of guessing it right within 3 tries is 0.03%. With 8 digits, the chances of guessing it right (if the sequence is randomly generated and not chosen stupidly) become so small, that if you would try to guess 7 billion peoples passwords, you would only on average guess 209 peoples passwords right. If characters and/or special characters are allowed the chances of guessing get even smaller, multiplied by a factor of 1^-4 or 1^-6 respectively.

Even if users are allowed to choose passwords and would choose them in a semi-predictive manner, within 3 tries it's impossible to crack any particular account and your average wouldn't improve to much, even if you would have a lot of data, like all birthdays in a family and the names of everyone in the family and every friend of the person. And you are also unlikely to have a reliable and comprehensive list of that kind for a large population.