My personal theory that I feel follows Occam's razor is that US and likely Israeli intelligence agencies had foreknowledge of the attack and either directly aided Al-Qaeda at certain points of their plan or indirectly aided them by not taking any efforts to stop them, knowing that the attacks would be an enormous boon for them to pursue their foreign policy goals.

Theories about the planes being drones/missiles etc. would imply a conspiracy of an unprecedented scale which would be incredibly difficult to keep covered up effectively.