Creative Speculation

When the planes hit some explosions were set off. Not clear why you need explosions if the plane impacted the towers, but we are deep into woo at the moment so I am going with it.
Well perhaps Aluminum bodied planes might not cut through enough core column steel. But enough charges could for sure.

The perps had a backup plan to ignite thermite at major columns at some point if the towers failed to collapse from the impacts to ensure the towers collapsed. Not clear why the perps had to have the towers collapse either, but again I've got my woo boots on snugly so I assuming this was critical to the plot.
It was part of the whole plan not a "backup" part.
Towers had to be completely demolished enough to allow quick removal & disposal of evidence. Can't have people snooping around in a still standing building with evidence of demolitions here and there. And had to have a huge spectacle to arouse the wrath of the people to get behind the oncoming Iraq invasion. A new Pearl Harbor event!

All of this was choreographed to make it appear as if no explosions or thermite were used, so ....... sorry but I've run out of rational thought.
You could avoid the snark but nonetheless
What may be "rational" to a criminal sociopath might not seem rational to a non criminal mind.