Who knows? There's a lot of cognitive dissonance out there. Nobody really wants to believe they live under a government that would murder or have them murdered in cold blood to start a war of aggression. Instead, they'd rather believe that their government is fair, just, honest, and working in their best interests.
This prevents people from looking at the 9/11 event and aftermath objectively.
It's a combination of several factors. A lot of people are yes, brainwashed. A lot of people are hamstrung by their own biases. A lot of people can't overcome their cognitive dissonance. A lot of people suffer from Stockholm syndrome. A lot of people work for or are dependent on the government and are incapable of biting the hand that feeds them.
My Mafia analogy is 100% unassailable because it's simply true.