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.
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.