How can you compete on quality when every delivery has been sitting in a box for ~10 minutes prior to pickup and ~15 minutes awaiting delivery? Pizza delivery companies like Dominoes spend $$$$$$$ to develop ingredients and methods that ensure that 25 minutes cold pizza is still palatable. That local hibachi or taco joint can't do the same. A cold, premium taco is just as unpalatable as the janky one from the cheap place.

They have no control over the rest of the experience: how it's plated and presented, how it's transported, the temperature it's served at, music and ambience etc. One pizza tastes much like another after it's been cooling for 30 minutes. Pretty soon, you don't even need a restaurant to cook the food, only a cloud kitchen. And why would cloud kitchens be owned and run by restauranteurs? It's much cheaper for the delivery company to hire a bunch of cooks to develop and cook menus for "different" restaurants that share the same facilities, suppliers and staff.