Consider the recent episodes of FBI whistleblower Sibel Edmond's "Boiling Frogs" podcast. The series I refer to consists interviews between Paul Thompson and Sibel Edmonds and Peter Collins.
The shows discusses various aspects of the relationship between US intelligence community and various members of the 'terrorist community' over the years.
Partway through the second show, Edmonds asks the guest several times why people don't believe it's possible that the US government would carry out a "false flag" operation on US Soil - implying that 9/11 could have been a 'false flag' operation. Her argument is that since the leaders in government (including the CIA) operate under a philosophy that the 'ends justifies the means', and have done false flag hits in other countries, then it follows logically that those same people would perform false flag attacks on US soil.
Many people only accept extraordinary claims when presented with extraordinary evidence. The theory that several people at the top of the US government deliberately murdered thousands of people, including their own colleagues at the Pentagon, falls into the category of an "extraordinary claim". That is why people require extraordinary evidence before they will entertain the possibility.
That is the answer to Edmond's question.
It's kind of bizarre to see someone educated and skeptical and intelligent, like Edmonds, adopt some of the theories of the 9/11 truthers. In fact the argument she uses is almost the same used by Alex Jones in his films saying that "9/11 was an inside job".
On the other hand. Edmond's show discusses several very interesting topics that are little discussed elsewhere. For example, the involvement of some of al-Qaeda in the Balkans wars of the 1990s against Serbia. The UAE royalty and the called-off bombing of Bin-Laden in the 90s. The CIA restricting the disclosure of information about the al-Qaeda meeting in Malaysia before 9/11. The situation with Alec Station, the FBI, and the CIA blocking communication to FBI headquarters about al-Hazmi and al-Mihdhar.
This last event is well documented - one of the FBI agents who worked in Alec Station is on a video recording saying it point blank, in the PBS show "The Spy Factory" (based on James Bamford's book The Shadow Factory). That is not a 'conspiracy theory', it really happened, unless the FBI guy and other people were flat out lying.
I have a book from the 1960s called "And none dare called it treason", by John Stormer. It is a ranting, extended diatribe about the alleged communist infiltration of every part of US society, from the churches to the government. It is also highly footnoted and somewhat well written. It is also clearly insane. For example - in one section, he starts screaming about the Korean war POWs who succumbed to torture and started saying things other than name, rank, and serial number. He calls these people all sorts of horrible names, casting aspersions on their character. Stormer, quite clearly, had moments in which he was a lunatic and an asshole. And yet, he poured out these tomes, endlessly, that were taken as truth by millions of people.
So. How do you know what is true? How do you know what is false? The idea that "9/11 was an inside job" is, essentially, a murder case. How do you prove murder? You need quite a lot of evidence; to convince a jury, motive and opportunity would also help. If the 9/11 truthers took their case to a jury right now, they would probably lose the case. In the United States, you also need to prove "beyond a shadow of a doubt" that the defendant willfully and knowingly perpetrated the crime.
Carl Sagan might advise to maintain a skeptical attitude - always demand evidence, and never trust proof by authority. Or, perhaps, proof by anti-authority.