Everyone was thinking "it was a controlled demolition" or talking about Lazer beams, trying to explain that the plane was edited in, maybe the towers never even existed and were invented by the media. But in reality it was a lot simpler, the Saudis sent people to fly planes into towers. Was the "jet fuel can't melt steel beams", like, purposeful misdirection or were conspiracy theorists just too imaginative for their own good
A lot of it was people with very little understanding of things like engineering, avionics, etc., grasping at threads and straws to try and find some smoking gun that proves it all had to be orchestrated, since the actual smoking guns (if they do exist) are all probably lost to time and secrecy at this point. I'd say a lot of it was, indeed, people being "too imaginative for their own good", as you said.
Also, a lot of the actual "boy that's fuckin' weird" coincidences that did actually happen are all tied to finance and industry, and a lot of people in the US were, and still are, very uncomfortable with imagining that the economic instruments that caused our ascendancy to world power could also be the driving force behind the levels of willful carelessness and cynical neglect that could allow the attack to happen, or allow people at the top of the food chain to swallow that bitter pill of thousands of dead civilians at home and far more abroad, for the sake of personal enrichment and power mongering. That's a lot messier and more upsetting for your typical reactionary conspiracy theorist than disguised cruise missiles, doctored video footage, or secret cabals with inscrutable, fantastical methods for world domination.
If the conspiracy is operating outside the norms of your society and ideology (through incredible conspiracy and magical technology), then you don't have to engage in any self-examination of the nation you live in, or your way of life. That's all fine, it's the Mysterious Other Outsider that has intruded on your otherwise moral and correct way of life, and that's why the bad things happened. Your ideology is safe. However, the moment you contend with the possibility that Capitalism and the financial system may have been a major driver of, at the very least, a lot of the questionable decisions that led to loss of life and perpetual war, then it becomes unavoidable that something might be wrong at the core of the American ideology, and your average person is going to have a much more avoidant reaction to anything that goes in that direction.