Legacy costs are why. You think "oh, it's just a big record store with some privacy controls, no problem!", but pretty soon you're trying to integrate some COBOL based system hosted on an IBM 360 to load data that is stored in a poorly specified format (mostly just text transcriptions of whatever idiosyncratic method that practitioner used--everybody did something a little different) on the order of a hundreds of millions of records. You may literally have to hire and army of data entry specialists to retype every single record in your new format, millions and millions of them. And you can't outsource that part due to regulations, they have to be housed in a secure facility that you need to build and get accredited (by who? It's unclear, you need to figure it out yourself. Oh, and if you guess wrong you're going to prison.).

And this is before the lawyers get involved with their reams and reams of regulations that all make your job more difficult. If you think you could just dump the database and load it on the new system think again, that's an unacceptable data risk, you need to broker all of the data through yet another system.