because a single plane is going more than 600mph, carrying full fuel tanks.
People focus on the fire/fuel burning temperature too much. The support columns were weakened severely by a plane hitting them, and didn't need to melt. They just needed to warp, and the fuel most definitely burned hot enough to warp them. They could support the building under normal circumstances, but take a few away and bend a whole bunch of them, and it's far from normal or ideal.
keep in mind this is an office building. A very tall one that was not meant to be hit by a plane.
And yes, I know about the architect saying they could take a hit from a 707. A 707 is a smaller plane, carrying less fuel, and it was under the pretense that the 707 was lost while trying to land. IE low on fuel, flying slow. Not fully loaded at full throttle.