Delivery has "locality of reference" (to borrow a term from computer science): if the density of customers is high enough, a delivery truck can travel to a particular neighborhood and spend the entire day on local streets, where the traffic isn't bad. The truck only has to use the crowded freeways to travel from the warehouse to the neighborhood and back. And it can do that during off-peak traffic hours -- few people need to have their groceries delivered at 8am, when everyone is on their way to work. When I visited LA, I was surprised at how empty the freeways could be outside of rush hour.