For the safety of regular driving mirrors and cameras seem to be enough. Even fully autonomous driving can be achieved using only cameras. Too much visual input (seeing a car and its surroundings from many angles, etc.) might be confusing for the driver. Parking sensors have sound alarms and visual clues to mark safe and dangerous distances, so visual depth is not necessary. Another thing is that having drones flying around each car would be complicated and dangerous in itself, they would crash into each other, disturb pedestrians, etc. I think in a not-so-distant future we might begin talking about "drone pollution" because they are used more and more extensively for nearly every purpose.