Automated cars on the other hand...
Also, I'd imagine that the drones would be (remotely) human driven at least the first few times they drop off a package, so they can learn where to drop off the package (similar to how Google handles training their self driving cars). After that, they could drop it off automatically, but hand off control to a human if the environment looks too different.
I imagine safety and power issues, talking to friends who research drone tech.