The problem is that withservice provided by a waiter in a restaurant, the total cost is at least moderately correlated with the amount of work the waiter does.

Sure, a waiter bringing a $15 burger vs a $40 steak doesn’t really impact how much work they are doing. But generally speaking, larger parties result in larger bills which means more work for the waiter. So a standard % of the overall bill is a good rule of thumb.

But that isn’t really the same with delivery drivers. Whether you’re carrying on box of pizza or three doesn’t impact how far you have to drive or how far you have to carry the pizza. You’re not servicing us, refilling drinks, cleaning up, etc. So whether I’m eating solo or have 6 friends over doesn’t matter. The cost is much more loosely correlated to the service provided.