There's two points.
1. How fucking hard is it to just tap the door as you drop the package off? Takes all of about 1-2 seconds. I think my guy doesn't do it because I'm on the second floor and he just hucks the packages up here instead of using the stairs.
2. Insurance.... Like I said, they could save a lot of time and money on insurance claims if there drivers where actual witness's to the delivery (if the person is home and they see them before they drive off)