Apart from the reasons usually given (fraud protection etc.) the main reason quite simply is that those systems are arcane and Byzantine. We're talking about A

Apart from the reasons usually given (fraud protection etc.) the main reason quite simply is that those systems are arcane and Byzantine. We're talking about AS/400 mainframes here with software written in COBOL and RPG (which actually was created for use with punch cards but still is actively used today). The preferred mode of inter-systems communication on these platforms is exchanging fixed-width files via FTP or socket services. Not only are these systems difficult to maintain or update, connecting modern systems isn't exactly easy either. That said, there's a whole lot of money to be made in this area in the next 10 years or so because everyone who still knows COBOL, RPG and AS/400 stuff in general is likely 50 years or older, which means there won't hardly be anyone to maintain those monstrosities any more 15 years from now. Many banks are currently migrating these systems to modern languages and frameworks, which for banks and finance in general mostly means Java. Unfortunately, that doesn't necessarily mean things will get better. There are many stakeholders in this game and their interests don't always align with the customers'.