Honestly, I would guess you've nailed it: The delivery company is collecting a fee because they are only paid for per delivery, so making a driver (and truck) wait doesn't otherwise make them money.

But there's no guarantee that the fees are passed on to the driver. Just in case, I'll pass a question back through my wife to see if some truckers are actually paid for waiting. They talk to each other; I bet they all know if anyone is paid to wait.