It may be hard to believe, but it seems that us Merkins just can't seem to accept that which is. Every national disaster - assassination of a beloved leader, terrorist attacks, even Katrina, seems to bring along an in-built need for a better, more complex explanation that what the facts support.
In philosophy, there is a principle called Occam's Razor. The idea is that between two explanations that are similar, the simplest explanation is likely to be correct. Popular conspiracy theories ignore this principle, instead opting for DaVinci Code complexity where more sensible, factually established arguments should hold sway.
Scenario: two giant airplanes loaded with explosive and volatile jet fuel crash directly into the centre of two giant skyscrapers. The jet fuel ignites, burns at 1800 degrees Fahrenheit. The structures are compromised because of a) the intrusion of giant flaming foreign objects into the structure and 2) the hollow-tube design of the building which is meant to resist lateral force, not the multiplied gravity and force of pancaking floors, each weighing several thousand tons, that give way as the cantilevers that were distributing the force between the compromised center columns and the compromised outside tube columns are heating to melting. And those elements don't have to melt to fail, either, just get hot enough to allow their load-bearing spec to be exceeded.
In short, most buildings are NOT designed to withstand missile attacks, let alone almost a million pounds of aircraft loaded with super-hot-burning fossil fuel. So, it seems that the likeliest explanation, which is flaming, heavy, high-speed missiles + structure not designed to resist those forces = structure failure. Versus what else? Surreptitiously planting literally tons of high explosive? Per building? With no one noticing? In two buildings? With security tightened after the '93 bombing? Huh? Doesn't pass the "makes no sense" test.
I believe that conspiracy theories like this are rarely proved but understandable. Kennedy was assassinated by a team of black ops people or the Mafia or the Cubans left floundering in the Bay of Pigs or maybe Marylin Monroe. Yet, years of examination and the collection of all the evidence seem to point to the simplest answer - a crazy dude did it.
We would all like a more diabolical and complex answer to "why" rather than the simplicity of a coordinated, low-tech attack. The feeling of being "had" needs more than such simplicity in order to be erased. Aren't we the greatest, strongest nation in the world, or don't we believe that anyway? Don't we represent good and not evil?
We may be Goliath after all and it's clear that others in the world see America and Americans that way. The truth hurts. The truth is that the Davids out there will be successful when they use the simplest means at their disposal - cheapest, most direct, easily done. And there does not have to be, and usually isn't, a sophisticated explanation involving archaeologists, popes, CIA operatives and evil geniuses. The only way to beat that approach is with a stick, literally. Or close the gates and stay at home. Either way, it won't stop nasty jerks from their bullying, no matter whether we're the ones being bullied or whether we're smacking kids upside the head for their milk money.
Welcome to the human condition. Now, get over it.