Restaurants are outsourcing the delivery to these companies. Many of the restaurants near me never offered delivery until Grubhub, Uber Eats, etc. allowed them to at little cost. The reason they didn't offer delivery before these options is exactly because of the economics of doing so.

It may turn out that the economics of the technology-middle-man aren't sustainable either. In which case they will have to decide between managing deliveries in-house or stopping deliveries.