How so? The delay between the hijacking and the crashes in the buildings for both planes were around 40 minutes... even if there were jet fighters ready to go at the time, the lack of knowledge of the hijacking being in progress for much of this time and the short delay make this kind of attack still feasible.
What was actually improved our chances to avoid such attacks are the limited access to the cockpit and processes pilots must follow in case of hijacking.
The measures at the airport are to limit the risks of hijackings to begin with.