What is true and what people believe to be true are different.
Is the Earth flat?
I think evidence says no. But there are "modern flat earth societies". So why do you believe some conspiracy theories and not others? Worth thinking about.
We started this whole thing by asking if 9/11 was a conspiracy at the level of what actually happened in terms of things colliding, exploding, burning, falling down, etc.
Meanwhile, the governments of Amerika, Airstrip One, and allies, with a record of lying even before this point,
To put this in perspective, how many cases do you have of world powers that have not engaged in lying?
...had this super secret intelligence that said otherwise, but which they couldn't show, and asked us to believe in it by faith. Useful idiots all over the world bought that.
Supposed Soviet military superiority was the fuel for a lot of military spending in the US, especially in the Reagan years. There was a mix of lying, self-delusion, and true belief. Along with lobbying from defense contractors and the military.
Governments have also had plenty of secret intelligence that was correct.
However they are formed, the opinions of ordinary voters matter too. Lots of Democrats approved of the war in Iraq despite private doubts for the simple reason that it was popular with the electorate to do things to avenge 9/11, and resisting Bush was considered a political liability. Sometimes the elite trembles in the face of popular opinion -- and Trump's hold on power is the most dramatic case.