It's a fallacy that because something doesn't happen frequently, that there must not be any attempts at all. Mind you, I agree that US security is embarrassing, especially compared to some other countries who use different techniques outside of the "in your face". But why would US security advertise every thwarted attempt? Sometimes it's better to keep things quiet, get some plants inside the network, make them think everything is okay. You get more inside info that way.
Or maybe we've just been lucky, and 9/11 was their last big push, and it worked. We were definitely a lot less secure back then, and had trained our passengers to cooperate with the hijackers to stay alive. Some things in retrospect that we changed do make sense, a lot like how you look back at someone who commits a murder or suicide, and realize there were signs that just weren't picked up.
Anyway, so many of the 9/11 denier claims aren't founded in good science, just like Moon hoax theories aren't, and that gets back to Freddy's attempt to try and get that ball rolling. Sometimes the simple thing is true...even if some parts of it seem unreal to us.
Some people also would like to believe in some bigger grand conspiracy, they feel that today's world doesn't have enough excitement, and they get an ego boost from thinking they are part of a small crowd of prosecuted believers who know some Truth no one else does.