i'm sure someone on here who actually researched can chime in.

Some amateur rager is not the same as coordinated hostage takers. Pilots hear a crying flight attendant pleading them to open up and maybe the terrorists claim they are pilots and then the real pilots think "okay, the terrorists just want hostages/money. We'll just open the doors". Not sure, I just assumed something like that. Also that the terrorists were trained by CIA (either directly by some "rogue" group in the CIA or just by proxy through bin laden) and maybe even had an idea of which pilot(s) were susceptible to hostage threat when planning it out.