Only one item can be delivered at a time. It can’t weigh more than 5 pounds. It can’t be too big. It can’t be something breakable, since the drone drops it from 12 feet. The drones can’t fly when it is too hot or too windy or too rainy.
You need to be home to put out the landing target and to make sure that a porch pirate doesn’t make off with your item or that it doesn’t roll into the street (which happened once to Lord and Silverman). But your car can’t be in the driveway. Letting the drone land in the backyard would avoid some of these problems, but not if there are trees.
Amazon has also warned customers that drone delivery is unavailable during periods of high demand for drone delivery.
Also, imagine drunk flyers in bad weather.
Ground traffic collisions can also cause collateral damage, but more often than not those are constrained to the roads or their immediate vicinity where not many people live. An aerial collision may happen above residential areas, and even slight fender benders may mean a double crash (…on little Timmy mowing the lawn).
Also, there’s no air bag in the world that can save you in a crash.
Road traffic is easy to direct and regulate with road signs, lanes, lights, painted lines. Good luck herding cats a hundred (hundreds of) yards above ground. It’s not a huge problem with planes because there are not as many of them and they fly at vastly different altitudes. Not the case with personal flying cars.
With ground traffic, you only need two blinkers (or two sets). Some drivers even struggle with using that two properly. Good luck for getting them to use more.
And that’s just the top of my head, I’m sure there are like 2634 other reasons.
They’re working on next generation air traffic control, that is automated and also can handle drones whizzing around next to flying cars, but developing that isn’t fast or cheap or deploy and will need extra equipment on the ground and in the cockpit.
…and that’s still just for the drones.