Correction: it’s more accurate to say that it’s less expensive for them to spend so much money maintaining the systems they currently have, some from the 1970’s running Fortran or even their own language, as opposed to doing a complete modernisation of their systems.
Because many legacy banks are financially struggling, or least just about coping, it’ll be an impossible task to modernise a legacy banking codebase and that’s what’ll need to happen in order for them to provide instant notifications.
I’m sure someone like @tristan might be able to explain the technicalities of it more accurately than I or perhaps correct if I’m wrong but this is just what I understand.