Why things are the way they are:

FedEx: Born as the original “multimodal transportation system.” In other words, they owned the airline effectively day one.

UPS: UPS started with using contract carriers. They then decided that control was more important than cost (UPS would argue that ultimate control saved money). They purchased one of their feed carriers, Interstate, and started UPS Airlines.

DHL: DHL opted for the multiple carriers approach.

Amazon: Same as DHL

What most people forget is the first (but not the only requirement) for a single carrier is common ownership. Amazon has a minority stake in all the carriers, so there’s no common ownership. Therefore, there’s no single carrier.

What some are hoping for is that Amazon decides to take the UPS route and change their model to support control over cost. IMHO, that’s not going to happen.