I would guess they save money doing it this way also. UPS used to charge more for residential delivery and this way they can deliver a load of packages to the local post office.
There is that too, having the post office subsidize the last mile. I am not sure that is more efficient since the package enters another package handling system.
Except that the other delivery services understand how to get packages to safety in theft prone areas (the nicer neighborhoods), and for some reason the USPS won’t do it, they take the packages back and leav those little notices. I hate that. I prefer the amazon trucks.