You know that feeling when something gets said so many times that people just start treating it as fact, no matter how shaky it was in the first place?
Like, Santa didn’t make the cut, but God sure did.
What are other examples of things that basically became true through repetition alone?
That would apply to a lot of entries of this list, which is a really interesting read: https://en.wikipedia.org/wiki/List_of_common_misconceptions
About 9/11 or actually any other major event: you typically get a lot of contradictory witness statements with so many people. The real picture only emerges when you have analyzed a lot of statements and looked at evidence. Thus early reporting often relies on a few statements that are found to be wrong later - perfect fuel for a conspiracy theory.