Drones let them ship things much faster to customers for cheaper than existing techniques and its easy to see how that helps their business. Margins of existing players in this space are typically much higher, which is something Amazon likes to go after (just look at pre-AWS hosting costs).

Maybe they're just better at keeping it a secret (given the parent comment by the throwaway id). If Amazon can figure out a way to get self-driving car aligned with a) Alexa, b) logistics, c) prime membership, and/or d) driving shopping to their marketplace - then they are obviously working on it.

Or maybe, typical Amazon, they're already working on Amazon Engine, in parallel with Amazon Wheels, in parallel with Amazon Windshield, and Amazon Steering Wheel, and we'll have something that can be used as a car in a few years.