It's all about ease/quickness of assembly at the factory (plus, the rivets don't ever loosen up and rattle like screws and bolts can).
GM FWD vehicles starting in the mid-80s had most of the crap inside the door pop-riveted in, including the window regulators! It is a pain the first time you see it.
You HAVE to punch out that steel center pin first - forget about trying to drill it with that pin in there. And as said, use a sharp drill bit. Clean out all of the loose bits from inside the door when you are done.