Because everything they do is connected to it. Obviously, they do not understand what kind of an undertaking it would be to install something new when the old thing is working just fine for the vast majority of people. What business has the time to do a complete changeover. GM used to have to shut their entire production line for over a month in order to make the changes for the new models.
That’s why some businesses are still running mission-critical processes on COBOL code written 40 years ago.