Why? Do you actually have any experience in this area? I do, and I can tell you, they do exactly that. Then multiple systems integrate with that mainframe, often using the user account as the unique identifier for the entire organization. Migrations are an absolute nightmare.
> Users have to go through that to do online banking, and the account system on that can be totally separate from whatever account system is running on the backend banking system.
It can be, but it isn't. Thus, the problem.
Honestly this type of "hardly believe" take is what every new employee right out of college (or myself 15 years ago) when they come up with ten thousand "simple" ideas for improvement without any organization, political, or system understanding. Then they act confused when their ideas aren't instantly implemented, because they don't even understand what it is they're proposing or why it is complicated.
Banks have been trying to get off of mainframes for 30-years or more at this point, spent tens of millions of dollars, but had someone just told them to "run a web server in front of it" this could all have been avoided.
