[aeturnum]

>Why does everyone say this?

Because the most popular theories about 9/11 conspiracies are never as simple as you outlined. I think a small group did plan and carry out the attack, and a part of the US government is just as capable of doing that as Al-Qaeda.

But once you start adding extra details (the buildings had explosives, the pentagon got hit by a missile, etc), the number of people you need "in on it" starts to climb. That is why so many people point out the huge headcount of the supposed conspirators.