Seemingly illogical on a per mile measure, the shipping is based on the 'Hub' model. Sorting and sub-sorting is done at centralized locations. Imagine the chaos if every item was sorted to its final destination by the pickup person, or at the local post office. Even sorted, the local centers would then need to route their work product to the millions of final locations.

The observation about the overhead costs of air transport is spot on...but each aircraft is at capacity to and from the hub to the regional centers, minimizing marginal costs. If each airplane carried a two pound package only, postage for it would be in the $100,000/each range. That efficiency is even now being exploited in the COVID vaccine distribution.

Federal Express pioneered the Hub system in the early seventies. They flew (mainly) business mail and small packages on a then-unthinkable next day delivery via a fleet of Falcon (like a LearJet) jets, in and out of Memphis; in by 2A, out by 5A, and in trucks at location in the PM at the latest. No one where I worked thought they would last 3 months, and none of us bought into the business, thinking it was stupid to send a package to Memphis to get it to the next town. We ended up working another 30 years to make up for our lack of foresight.

JE