That a conspiracy occurred on 11 September 2001 is a well documented fact. I don't believe that any rational man or woman could, with all the facts, accept the US government's (i.e. "official") version of this conspiracy. Such a man or woman, knowing all the facts, still accepting the US government's version, is accepting this version for reasons other than facts.
What people say are also facts - that is - it is a fact that they have said something, irregardless of whether what they say is true. Let me encourage you not to disregard witness testimony, as this was also important in the Old Testament age (albeit no one could be put to death on the testimony of only one witness, so at least 2 witnesses are encouraged to establish a matter). Look also at the eye-witness testimonies for the 911 event - numerous witnesses reported hearing explosions in the buildings - why disregard such valuable evidence?
Finally, if you believe those who say the Earth is flat are ignorant, how do you explain that on clear days, people can see further than where the curvature of the Earth should allow them to see? I don't think the bible rules out a flat Earth, and in some passages, expressions would be easier to understand were the Earth flat (e.g. Rev 6:14 "And
the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places.")
I understand your response toward this topic, as this was also my response when I first heard people even had such thoughts. But after reading some of the evidences why they believe as they do, I no longer think of them as irrational or crazy, but rather critical thinkers, or at the least, conspiracy theorists.