For years that was how delivery operated in NYC, and in fact, in NYC there was Seamless.com which was an aggregator, but deliveries were handled by the restaurant.
That model was vastly preferable, as delivery people were familiar with the neighborhood and you wouldn’t find yourself accidentally ordering from a restaurant on the other side of Manhattan that will essentially guarantee your food will be cold.
As far as the number of engineers required, I think you vastly underestimate the tech and infrastructure required. Sure a few dozen engineers could build the core matching and routing engine and the restaurant and delivery person apps, but all of the scaffolding around that is huge. Who does customer support? How does the customer get ahold of the delivery person? What happens when an order gets misdelivered? How do you deal with the various regulations in different municipalities around pay? Heck, paying people is a few dozen people alone.
And then you have to think about supporting all of these things in places where the delivery models might be totally different- car delivery is totally different than walking delivery. What about users whose orders are late? In some places they have health regulations that preclude delivering food that has been waiting for over a certain time. The list goes on and on.