While they do rely on COBOL and old mainframes a great deal, that isn’t the only software supporting the company and its operations. That fact doesn’t negate what I’m speculating would be the cause.

These big banks have multiple programming teams that use different programming languages and work on different products.

If you go to their careers page, you will find tons of Java, .NET, and Python jobs posted. I’ve never seen a COBOL posting at a big bank (which doesn’t mean it’s never happened, but I can see any of these more modern languages posted any given day).