Let's say you hire drivers as employees and pay them $15/hr plus tips and reimburse them for mileage. You charge a $4.99 delivery fee. Drivers work set shifts and are paid hourly whether they are making deliveries or not.
That means each driver needs to be making at least 4 deliveries an hour or you're losing money. That's not even really counting for mileage or any other benefits like health insurance or retirement (not that jobs like this usually provide this, but people seem to think that they should).
When I lived in DC, driving anywhere could take at least 15 minutes. Getting 4 different trips from a restaurant to somewhere reliably every hour would be difficult. Obviously, drivers can pick up multiple orders and take them in one round trip, but you're at the mercy of what orders happen to come in and where they happen to be located. It seems like it would be very hard to make that sustainable.
Of course, Domino's and lots of other places do it, but they probably aren't paying $15/hour and they also have one central location and more predictable demand. It's more feasible if drivers always go back to one central hub rather than having to get orders from random different restaurants all over the city.