Doug, my understanding is that if one ordered through the delivery service site, the restaurant gets dinged heavily for the service. By ordering direct with the restaurant, they don't have that fee. They, of course, can choose how to handle delivery, either with their own folks or by contracting like you mention. It's not just restaurants that do the latter...PetSmart uses DoorDash for delivery, with or without a fee depending on the specific order, but they do have provision for tipping the driver.
--
The most expensive tool is the one you buy "cheaply" and often...