Indeed, why hasn't it happened yet?
You would think there would be any number of Iraqi or Afghani nationals and sympathizers living abroad with access to the sophisticated infrastructure and possess the knowledge required to carry out one of these attacks.
So, why hasn't it happened yet? If it's as easy and devastating as you assert, why isn't it happening all the time?
In my opinion, it's not as easy as you suggest, and the personal penalties for getting caught engaging in "cyber terrorism" far outweigh the potential damage you can inflict.