Do you know why medic-vac helicopters exist?  Not necessarily because they move faster than an ambulance, but because they can travel as the crow flies.  Even if a drone and a human delivery driver cost the same, a human could never come close to the delivery speed of a drone simply because they can fly over stop signs, stop lights, inefficient road design and get to the home the fastest.  And for Amazon, that is the value, getting the package to the customer as quick as possible.