[Confusion]

  It seems that no one who brings this argument up 
  acknowledges the possibility that terrorism doesn't seem 
  like a big threat precisely because of the extreme 
  precautions taken to stop it.

Then on the one hand, the FBI and CIA would regularly, and very publicly, celebrate the apprehension, or otherwise prevention, of would-be terrorists, with some non-detailed description of the realistic terrorist attacks they were planning to undertake.

It would increase morale at those agencies, increase their standing with the public and discourage other would-be terrorists. Not much is given away if we are told the 4 people apprehended 'planned to plant a bomb at a plane in JFK airport by sneaking past security'.

Instead, we only get the occasional apprehension of delusional people with ludicrous plans.

On the other hand, if there really were many terrorists with actual plans, there would occasionally be a successful attack. Even at an unbelievable 99% prevention ratio, some would come through. An illegal immigrant loner building pipebombs in his cabin-in-the-woods could easily escape detection.

The much simpler explanation is that there simply aren't many terrorists with realistic terrorist plots. If anything, this surveillance should be defended by appeals to 'stopping the next mass shooting': that's an example of a crime that occasionally happens, while also many are prevented.