1) It wasn't fire alone, there were two aeroplanes that hit them if you remember.
2) A fire fuelled by something like jet fuel burns hotter and faster than a fire fuelled by something like carpets and wood.
Try comparing it to the other times that skyscrapers have been hit by jet planes rather than just having fires in them.
Like this:
When an aeroplane hit an apartment block in Amsterdam. Imagine if there were floors above the impact point, what would be holding them up now?
And remember, this is a concrete building, and concrete is fire proof. The WTC was a steel framed building.