Because Americans are brainwasned and will believe authority by virtue of their authority. And nothing else.
I'm afraid that's a very unconvincing reason to accept your preposterously complex version of events.
Firstly you haven't made any case for America being brainwashed to blindly accept authority. The ultimate authority on 9/11 was G.W.Bush. Are you saying Americans believed everything he said? You don't recall there being swathes of Americans of other political leaning who thought he was an idiot? Yet you claim America was brainwashed to accept authority blindly.
But I can hear the paranoid calling out "No, of course GWB was not the
real authority. He was just a puppet.
'They' were really in charge."
Sadly that doesn't help your claim. You can't say Americans were brainwashed to accept the authority of some nebulous group which nobody knew existed never mind believed to be
in authority.
Anyway, none of this explains why 'they' had to go to the absurdly complicated lengths you describe to manufacture a fake 757 crash site.
Scenario a): 'They' had brainwashed America to believe them.
In this Orwellian scenario, why fake the crash site at all? Just announce that the crash had happened. Say it crashed in a lake. Or in the ocean. Everyone will simply believe you, right?
So in this case, there is no need to fire off a batch of not-yet-in-service missiles, with only a modest chance of making the correct impact scars, then seeding the impact point with debris from a real 757 crash (oh, yeah, about that - obviously you still need to stage a real 757 crash to generate accurate-looking 757 debris). Nor do you need to suppress the real radar, eyewitnesses, cellphone traffic, flight data etc.
You've created a virtually impossible job for your army of henchmen to contain the situation, and all quite needlessly. If you proposed this at NWO headquarters, you'd quite rightly be fired. Your seat at the big table would suddenly drop into the floor...
Scenario b): No, actually they hadn't brainwashed everyone.
Same deal, except you have to make every detail consistent for the legions of un-brainwashed folk who will encounter parts of the evidence and spot anomalies. In this scenario, your plan makes even less sense. You need a completely plausible crash site, and it has to tie up with radar, radio traffic, eyewitnesses etc.
At NWO HQ you propose your cross-fired missiles and convoluted plan to suppress the real trail of evidence and dress up the site with debris from a real 757 crash. After an awkward silence, someone says "So we still have to crash a 757 anyway?"
"Yes, plainly we have to have credible debris."
"Ah. Only, why is it better to transport all this debris to Shanksville and then bury most of it (which will really mess up the nice crater our missiles made, by the way) instead of just, you know, crashing it at Shanksville?"
Number One will stroke his cat and his finger will hover over the button for your chair.