Because it's easy, cheap, quick, and hard to trace. And the payoff can be very big. In most companies, when someone dials 9, they get an outside line, then if they dial zero, they get an outside operator and access to long distance service. I need you to punch a bunch of buttons for me. They're going to get you to punch the buttons 90 transfer, and you'll have transferred their call out.
People fall for it.
The guy who had the job before me kept lousy records. I have no idea what kind of parts you need, I can't keep them on stock. Do me a favor. Tell me the make and serial number on that copier. Well, people are helpful.