They probably 'knock', just not very loud - and they don't wait long either. I can easily hear the doorbell or a loud knock from my home office. Whenever FedEx or UPS comes buy I only know they are there from the dogs barking, never actually hear the knock and they never ring the doorbell.
Had a scheduled pick-up on a miss-shipped item. First day, never heard anything - just got the note. Second day the dogs went nuts, so I looked out the window, saw the truck, picked up the item and walked downstairs. Driver was about to stick the note on the door and walk away by the time I got the door opened. So one light knock, no attempt to ring the doorbell and no second knock... Seems to be much worse with their 'rental van' drivers as opposed to the real FedEx truck drivers.