Road gets you everywhere, rail only gets you to the nearest terminal. Depending on the signalling system used rail can have a higher capacity from terminal to terminal, leaving the 'last mile' (which in reality is more likely to be the last 100 miles) to be travelled by road. Road needs more maintenance and the work itself is more labour-intensive.

Imagine a package-switching network comprising an (electrified) rail backbone with roads spreading from terminal nodes to the destination points, fully automated package store-and-forward, electric self-driving trucks.

This would cost loads of money and loads of jobs. It might also solve parts of the problem. Maybe Amazon's successor is already thinking of something like this?