The same reason why ACH transfers still happen by transferring large files to a FTP server once a day or why San Francisco's train routing system still uses 5.2

The same reason why ACH transfers still happen by transferring large files to a FTP server once a day or why San Francisco's train routing system still uses 5.25-inch floppy disks or why social security payments are calculated by a system running COBOL. It has always worked this way, and there's no political will or public pressure to change it.