There would not be such a case that no one wants to do the delivery. People are motivated by profits and if they want the profits and they know that the delivery is a step to that they will do it.
If the money earned from delivery is more than pizza, they will do delivery.
When money earned from pizza=delivery they choose either
When money earned from pizza> delivery, they choose pizza.
Realise that if there is more or less money earned from doing one thing to other, there would be a change in jobs. So if suddenly more people start doing delivery, there would be more supplies and pizza would become more profitable. Vice-versa.
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