In many cases (e.g. banks) these systems have been running for decades, they're often badly documented and have had a lot of fixes applied for issues that arise.

Re-writing that, is going to be a nightmare, as you've got no spec. to work from and mistakes may not show up immediately (think monthly or annual payment processes)

It's a very high risk endevour with little immediate reward for the team doing it. If it goes perfectly, no-one notices, if it goes badly it could seriously impact your whole company.