As bad as air travel is in absolute terms the way they cram people in it's not too bad on a person-mile basis. About the same as a typical car being driven with two people in it.
True, cars are very bad for climate change as well. I think that the reason people focus on airplanes is that they tend to take you a lot further. Almost nobody flies for a 100km trip, like very few people undertake a 10.000km car trip.