Shouldn't this have happened? 

Sorry benFun7,
steel frame skyscrapers have only fallen 3 times in history.
You see, it's scientifically impossible. Steel cannot melt or even bend at the temperature at which jet fuel burns. And in the pic you show, World Trade 7 which was never hit by a plane but fell anyway, there was no plane or just fuel. Just two small fires.
On Sept 11, 2001, the laws of physics were violated 3 times.