I think it's a combination of several factors. One is that few people join or leave a bank because of their website. It's usually encountered already after you've deposited your money and it's going to be very inconvenient to leave them.

Another factor is that they typically have a big pile of money budgeted to develop it, which means hiring large teams, which means finding work for many people, which means building a very large complicated website to have something to show for it in the end.

This is where design-by-committee comes into play, which is a third factor, and where all those asinine rules come to play. And don't forget, they are already under the authority of all the regulatory committees, with a committee deciding exactly how to follow another committee's rules.