Remember...the fire didn't need to get hot enough to melt the steel, but just hot enough to bend it beyond load-bearing limits.
But the fire was only contained in the upper stories, why would the entire structure below it 'collapse' when it has been successfully bearing the entire weight of the towers for decades?

Ever seen a concrete block explode? I have. The gasses inside expand tremendously when subjected to fire.
Yes, but what about the steel core of the building, why did that 'explode' into a million pieces? By the way are you a physicist, structural engineer or architect?