Er, no.
You don't just magically hack into computer systems, like something out of a cheesy movie. There are specific methods of attack that hackers use, like bad passwords, SQL injection, social engineering, buffer overflows, privilege escalation, etc.
Implying that banking systems are just waiting to be opened like a can of beans is wrong.
What is insecure, for example, is logging into a banking system on a computer that you don't control, because it's possible to strip the SSL and install a private security certificate to proxy between the browser and bank's server.
There are lots of ways for banks to get compromised, e.g., personal data being stolen perhaps as part of offshoring projects, but they would be specific ad hoc exploits used at some point, and not just left as an exercise to share with friends on mumsnet.