What's important for humans is whether spending time in an airplane or in a car is more dangerous and whether choosing to use an airplane or a car for a trip is more dangerous. In either case the airplane is safer than the car.
Most car crashes occur near the home, at distances that make air travel impractical. Who drives to the airport to fly 3 miles? Comparing "car miles driven on trips" to air miles would be more enlightening.
As was explained, it is a function of the number of takeoffs and landings. Therefore it only holds because airplane trips tend to be a lot longer than trips with other vehicles.
