They may not have a literal line item on an internal spreadsheet corresponding to how much of each bill goes towards delivery, but they are still paying for per

They may not have a literal line item on an internal spreadsheet corresponding to how much of each bill goes towards delivery, but they are still paying for performing the delivery. When they outsource this service, they pay a third party for doing it, but save on the costs of doing it themselves. When you consider how expensive the third party is to the restaurant, you also have to consider how much they would be spending if they do it in-house.