Why traffic signals are not smart?

One evening, I was walking on the footpath in Chennai. Followed behind me the sound of an approaching ambulance, alarming me about its urgency. Unfortunately, the signal turned red. How long will the ambulance be stuck here, I wondered. Strangely, the motorists ahead of the ambulance decided to drive, despite the red light. To my surprise; motorist around the other signals (even if they were green) froze!

I found this fascinating that even though the signals were not intelligent enough to detect an approaching ambulance, the motorists did. 

I thought, why traffic signals did not communicate with each other about the approaching ambulance. 

Why were they, not smart?