Terrorism doesn't respond to deterrence
If it did, then Israel would be safe. Terrorism isn't deterred by good police work, either. Good police work is essential to stopping some of the terrorist attacks but it's unlikely to get them all. Again, Israel has some of the best intelligence agencies and police around and they're still attacked.
Terrorism is more like a cancer of humanity. Cancer doesn't respond to deterrence - it has to be tracked down and fought before it spreads and kills.