Why secure airports when it's planes that fly?

With all the debate going over TSA's naked scanners and intimate pat-downs, I think one simple question is being totally ignored.

Why secure the entire airport when it's only the planes that fly in the sky. The reason we need added security for air travel is their unique vulnerability as compared to other forms of transport. An accident or attack in mid-air can lead to nearly 100% fatality. Also planes can be used as weapons in even more significant disasters as demonstrated by 9/11. So the objective of securing air travel should be to keep the flying objects safe. Anything and anyone that boards the airliner should be thoroughly checked. But why secure the entire airport?

Airport is tens of square miles of area with hundreds of access points. To try to secure this entire area is to make the task at hand enormously difficult than it has to be. There is nothing more special about an airport than is about Grand central railway station. A terrorist group is not going to achieve anything more significant by blasting a bomb at airport gate than he would by blasting it on a railway platform. Consequently, security of airport can be as tight (or loose) as that of a railway station. All the extra vigilance however should be concentrated at the gates where passengers and crew board the airplane - the real vulnerable entity that needs protection.

It is a matter of common sense that smaller the area to protect, more effectively it can be done. Why not apply that to air travel security?

Moreover, securing entire airport varies from country to country. Flights fly to US from all over the world, in multiple hops. Securing the safety of a flight bound to US, then depends on securing the boundaries of all the airports where the flight was boarded. Imagine how behemoth that task is and how easy to breach. Establishing the checks at the doors of the actual plane simplifies that problem by magnitudes.