Most of these organizations can't tell the difference between competence and incompetence, so any money they spend on IT is likely to be wasted. The more ambitious the project, the more pointless it is.

Maybe it's different in other countries but certainly in Australia, banking is a heavily regulated industry, so replacing a banking host (or even parts of a banking host) would require lots regulator approvals and sign-offs (regulator has to sign off all of the technology and the code that will be run - for existing systems, all changes still require approval, but the initial hurdle of approving something new was dealt with a long long time ago).

I suspect it's less of a technical issue and more of a people issue. Nobody wants to take the responsibility if it fails because of some undocumented/unforeseen behavior that only happens in production