Count the number of dead cars on the roadside tomorrow on the way to work. Now imagine each of these cars falling 500 meters onto a random part of your favorite metropolis.
The reason we don't have this problem with light planes has mostly to do with low numbers of planes and airports. Airplanes are expensive, and pilot licensing rigorous. If any driver could be a pilot, and any car could fly, there is no amount of engineering that would make up for slack maintenance, equipment age, and operator carelessness. Short takeoff car-planes won't glide like light planes. Without power they will become bricks. Big bricks full of gasoline (or empty) falling on city cores.
Even if we could figure out the engineering to make such a vehicle workable under best case assumptions, this is still an idea that will never fly.