You are exactly right on this point. Why does it take an enormous degree of sophistication to hijack four aircraft? However, they would probably need to know how to fly the planes in case the pilots were uncooperative.
You don't just waltz in an steal an aircraft. There is training, intelligence, surveillance, planning and more training. Then there is the support infrastructure: Safehouses, vehicles, cover stories, equipment, communications - A lot of logistics.
And all of this must be done clandestinely. This is not as easy as it sounds, even in a relatively open operating environment like the U.S. - This was not an easy operation to mount.