Frankly, people are just rather dense and think they know things they do not.

Airliners can achieve high subsonic speeds and are fairly maneuverable. The bolts on the engine pylons shear off on impact with the ground and the plane disintegrates on impact as it is largely aluminum composite sheeting. The fuselage acts as a ablative penetrator - jabbing with a rolled up sheet of paper, in a sense.

The "no plane" theories were an early poisoning of the well done to distract from research of the connections behind the events and who should have acted on intelligence reports.

IE - the CIA got everyone arguing about whether or not there was a plane so that we all missed how the CIA set up the operation and made sure it wasn't intercepted.

Since most people do not have much experience with reality, it is easy to distract them with physics arguments.