LC, there is a distinction here I'm not sure you're grasping.
Basically, it is far easier to find a weakness from the outside to exploit (i.e. our self-assurance that no one would attack using such low-tech methods as the hijackers employed, or our assumption at least that no one would; much of our defense was against more high-tech methods such as cruise missiles or fighter jets, to name just a couple of examples) than it is to plot to do something from the inside, simply because the outside version allows for less people knowing about the plot to begin with. In order for this plot to be carried out from the inside in such a way that it makes it seem as though it occurred from the outside, it would require tens of thousands of people with at least SOME knowledge of the plot (i.e. they may not know about the whole thing, but they knew about part of it) to carry out, whereas an attack from the outside carries much less risk of being found out and halted ahead of time and requires much less people; the materials are already there and waiting for them to use (the planes are legitimate planes and don't need to be painted or altered to look like AA or UA planes, short knives were allowed on board at the time, airport security was much less stringent, so it could have been possible to smuggle something on board that resembled a bomb (enough to fool the passengers if they didn't get a close look, assuming they were even looking and not too scared to do so), flying lessons were easily obtained, and most importantly, we weren't looking for such a low-tech approach to an attack). That's not to say every outside attack would be successful or even be carried out; sometimes we do get lucky and stop things ahead of time, but it carries a greater chance of success than an inside attack of the magnitude required to carry out this job at least would.
Do you see now why the vast majority of us perceive an inside attack to be ludicrous? Even if the tens of thousands each only had one piece of the puzzle to carry out the attack, all it takes is for one or more to put the pieces together after the attack is complete and be brave enough to risk their lives (hyperbolically speaking) to get the information out. In nearly seven years, there has been NO ONE. Deep Throat came forward even before Watergate was complete, and that was a vastly smaller conspiracy; I find it hard to believe that not one person in tens of thousands would have a similar reaction.