That’s not why they sprint back to their truck. They leave quickly because they are being timed. There is a GPS on them and their managers know exactly where they are at all times. They are timed from point A to point B. It is recorded when the truck stops and how much time is spent at each place. Very little time is given them and they are penalized when spending too long at a house, even down to the seconds.
Disclaimer: I do not work for a delivery company, nor do I know anyone who does. I have just seen various delivery people clicking a clock on their equipment.
IOW, like doctors these days.
I knew some UPS guys who mentioned that at some point they all had nightmares about their deliveries - timing, wrong package, can’t find the address, etc. Kind of like dreams about needing to take an exam but you can’t find the exam room. : )