Why they could not use real planes

They could not use real planes on 911, because,  in the planning stages any use of real planes presented real and insurmountable problems.  

Just try to imagine the planning session.  The narrative will be that terrorist skyjackers,  sent by OBL would takeover jets, fly them to the targets then crash them.  

Problem:  Jet crashes would not take down the buildings.

Solution: Load the buildings with explosives sufficient to demolish them.

Problem: With the buildings loaded with explosives, both planes must impact the towers and cause maximum damage, such that it is believable that the crashes, not the planted explosives, cause the buildings to fall.

Problem: Real people, even suicidal terrorists are unpredictable.  They might not accomplish their tasks for a wide variety of reasons, many of which are not under anyone's control.  Loss of nerve, bird strikes, equipment failures, passenger revolt/intervention, failure to overcome the flight crew, just to name a few.  If even one plane did not strike it's target, or if both planes struck the targets but so obliquely that minimal damage was done, the entire operation is exposed to failure.

Solution: Fake the use of planes then fake the required impacts.  In only this way could you be absolutely certain that the required scenario would be perfectly accomplished in the required manner.

Because news media is often fakery, newscasters cannot challenge their own news/video feeds being played over the air, by people throwing switches in the control room.  "Normal" news fakery is often done, simply to give viewers an enhanced view of the news.  It cannot be let out that this is done because the media would lose it's credibility.  But, to say that something must be so, because it was seen on tv,  is ridiculous in the extreme. Many things seen in movies and on television are fake, but seem real.  The viewers only clue in many cases, as to what to believe or not, is the context it is given.  Thus, if you are watching a movie or story,  you know that it's special effects.  If it's presented as news, however,  you would have no way to know that what you are seeing is either special effects or real live footage.  Only later analysis can reveal the differences between the real and the false, and sometimes not even then.

But this was a massive project with too many components to make them mesh seamlessly,  so there are a plethora of artifacts that give the mission away.  Artifacts that would not be there if all of these things had taken place for real.