While using a regular truck is likely much more cost effective than having a specific one (less investment, easier repairs, ...), each design may be optimal for some situations: some trucks have the size to load pallets, some are equipped to load big/heavy stuff, ...

Most regular vehicles are not optimised for many/small/various packages and are not optimised for high frequency in/out and searching/unloading as mail trucks would need. You'll easily see how the package/postal delivery drivers loose time searching the packages through the back and energy bending, opening/closing doors when they use such regular trucks.

One of the great things here is how they seem to look for best posture and efficiency for the driver: it's not only about speed of delivery, but likely to also prevent accidents during the trip, injuries from the work, ...