It is also easy to bypass, as small or private airports are not subject to TSA scrutiny. A criminal could load up a small plane with, say, explosives or biological agents and take off from a tiny airfield outside of TSA jurisdiction and then fly it into whatever. And this could happen at a dozen locations for maximum saturation. And it completely bypasses the "security" of the big airports. Hell, he could drop chemical agents via a crop duster onto a busy highway during rush hour, and no one at the TSA would be the wiser until it's too late.
Add to that certain types of airplanes that don't even need an airfield. Sure, they don't have as much range, but they've been around forever and a day and are reliable as hell.
Old bush planes are still sometimes found in Manitoba and fixed up good as new when they were built in the '20s-'40s, and can take off from any farmer's field.
