Because the idea that airplane travel is safer than passenger vehicle travel is propaganda/marketing. Flying is safe, but in practical terms the sentence "flying is safer than driving" is not only nonsensical but also false.

If you could magically replace every passenger vehicle trip with a commercial airline flight, including the quick trips down the street to the grocery store, there would be more fatalities. Because, statistically, flying is less safe per trip.

The reason why flying is safer for long trips is because the "danger window" of a flight is generally takeoff and landing, while the "danger window" for driving is basically the whole drive.

You can slice this another way. In the book Freakonomics, they discuss that "per hour spent traveling" the passenger vehicle and the airplane are approximately the same risk level.

It doesn't, it's part of the mental gymnastics people are going to through to try to argue that commercial aviation is more dangerous than it is.