Banks have decade old hardware and take forever to upgrade software and protocols. You expect them to design a sophisticated attack and run it for however long. On top of that, it would be really easy to find out which bank was doing it and that has huge legal and customer based ramifications. Definitely not worth it.
Lol, we a major consumer bank here that have "system error" and everyone's payday salary is mixed up. I imagine some miserable dev tasked to run a batch script and forgot to do some steps.
For sure these banks wouldn't use their own hardware, but hire a new team, new hardware etc.
