They do this automatically, without ever investigating or questioning for themselves, simply trusting in whatever narrative is provided to them via mainstream media. This is partly due to fear — it's easier, and more comfortable, and less frightening to believe there are enemies far from home who wish us harm than to consider that the people we chose to put in positions of great power here in our own country are manipulating us, and lying to us. It would definitely bring up some cognitive dissonance, and this is why the "conspiracy" reaction is still prevalent, because there are some who believe that ignorance is bliss.
More Than 165 Million Americans Don't Believe The Official 9/11 Story and Think the Government Is Covering Something Up
The latest polls show that 54.3% of the American population doesn't believe the official story that their government told them, and that they "strongly agree" or "report" their belief that something else actually happened on that day.