UPS employees are unionized, and driving routes (vs. working in the warehouses) is a sought-after position that is one of the benefits of seniority (as I understand it). I expect that they might have issues rolling out a computerized delivery fleet.
(Not making a value judgement here. Airline pilots have a tough job and do it well, and deserve to be well-paid. But the industry is not as nice as me.)
An airline customer will switch carriers for as little as a $5 savings on the cost of a multi-hundred-dollar ticket. There is essentially zero customer loyalty. If you can't convince customers to pay more, and all possible cost-cutting measures have already been implemented, then you have no room.
In an ideal world, a business could survive forever in an environment like that. In the real world, business is variable, and with no margin, you can't build any resiliency. A couple of bad years is enough to screw you.