Ground control is much easier to secure. Instead of having to find a needle (hijacker) in a haystack (the millions of random Americans flying each day) with a 90 second search, you can do proper background checks on the small number of people who are allowed to be there.
Until one of those people turns out to be malicious. There is no way to tell what anyone is thinking, or whether they've been spending their time outside of work being slowly corrupted by certain influences.
And yet the rate of terrorist hijackings, although tremendously small, is much larger than the rate at which secret service agents betray the president.
