I tend to place conspiracy theorists in the same area as alien abduction 'victims'. People are just attracted to something more glamorous than reality.
A bunch of dumb muslims are manipulated into hijacking four aircraft and causing mass destruction? Can't possibly be true. Airport security isn't that lax. It can't be that easy. It can't possibly be that easy. The reality of course is that some things are much easier ONE TIME than ever again. There are countless examples, especially in wartime, of how a few people did something audacious but somehow because it was American planes flying into American buildings? It couldn't be that easy. The alternative is much more glamorous.
Same with the Moon landings, although more tricky because there were multiple landings. Ignore the decades of work that built up to them. Pick one and say it couldn't possibly be that easy. The alternative is much more glamorous.