It doesn't have the same publicity quality, though. The groups that carry out these attacks do it for attention, and they're going to do whatever they can to maximize their press time. That results in worldwide publication of their message, often requiring their opposition to acknowledge and respond, and gives them outlet to advertise to potential new recruits. Airplane attacks and crashes get a lot more coverage than incidents involving similar loss of life due to the emotional factor of being isolated 35,000 feet in the sky, and that makes the cost involved in launching a more intricate attack on aircraft worthwhile.
You can try to run away from a bomb at a parking garage after the initial explosion and an ambulance will be there to check things out and rescue people pretty quickly. If you're in the plane when it gets attacked, you have no recourse and virtually no chance for survival, which is not usually the case when a ground target is bombed with conventional explosives. Even if you survive the actual ground impact (zero chance if an event happens at cruising altitude, small chance if it happens at a more reasonable altitude during takeoff/landing), there's no guarantee you'll be anywhere near civilization that can give medical help or attention. This makes the risk of an airborne attack much more frightening, since death is virtually guaranteed.
Passengers want a reasonable assurance that there's something more than chance protecting the flights they're boarding (and for whatever reason, they don't usually make the same types of demands from ground entities like parking garages or bus stops). That's why the airlines added their own security checkpoints in the 70s as hijackings became prominent. That's also the reason the TSA can exist at all. The TSA takes it too far, and we all acknowledge that some threats can't be screened out, but just going back to nothing isn't a plausible solution, at least not at first. There needs to be a more gradual transition if we're ever going to get there.
