I've actually thought about this exact point very hard for a long time.

There is only one thing known for certain: a group of human beings murdered thousands of Americans in cold blood that day. Either it was 19+ Muslim hijackers, or US deep state agents, or some combination thereof.

Like you I also once considered it difficult to imagine Americans murdering their own people like that. I presumed that out of a reasonably sized group of conspirators that at least one of them would grow a guilty conscience and foil the plot. So I asked myself: why couldn't a Muslim plotter grow a guilty conscience in a similar way? Why was I so much more willing to believe that Muslims were the cold-blooded murderers? And I couldn't give myself an answer that didn't sound at least a little bit racist. I realized that part of my mental barrier was constructed from the decades of propaganda dehumanizing Muslims that all Westerners like myself are subject to.

I found an even more complete answer in the form of the shattered assumptions theory. It's a psychological model that says that one of our basic assumptions is that the world is benevolent, and that experiences which shatter that assumption are the root cause of trauma.

Once one has experienced such trauma, it is necessary for them to create new assumptions or modify their old ones to recover from the traumatic experience. Therefore, the negative effects of the trauma are simply related to our worldviews, and if we repair these views, we will recover from the trauma.

9/11 was a deeply traumatic event which shattered our worldviews. Before 9/11, most Americans would have had a hard time believing that any person---even Muslim people---could be capable of such evil, but we were forced to collectively accept that fact by the horror we witnessed with our own eyes. In that crucible of collective shock and grief, we forged a new worldview that included a terrifying new threat to our lives: radical Islamic terrorism. Accepting this fact repaired our views and allowed us to recover from the trauma. However, once trauma has been repaired in this way, any differing explanation threatens to shatter that worldview all over again and re-traumatize the individual.

In other words, fundamentally altering your understanding of a traumatic event creates cognitive dissonance. That is why I believe it is easier to accept the "they let it happen" explanation, because it leaves the initial threat model intact rather than replacing it. It's also more disturbing to accept that your own government hurt you, just like people are reluctant to accept that their parents were bad. It's worse when it comes from inside the house.

to me its way easier to know something is gonna happen, and only a tiny tiny group of people in on it can influence others to move a few pieces around to ensure it happens. Its really hard to keep people quiet without killing them. Its easier if they don’t know they’re involved in a conspiracy

Intelligence operations are run in such a way that mission critical details are kept to the smallest group of individuals possible. People are only told what they need to do, not why.

america bungles everything lately

Does it bungle shoveling money to the 1%? America has always been good at what it really cares about, i.e. enriching capitalists and sabotaging socialism. Remember how fast that Trump tax bill passed?

I find it hard to believe that americas got people that ready to kill citizens all willy nilly.

The psychopaths who work at the CIA really believe they're doing all of it for the greater good.