That is true, of course, in a normally peaceful environment. But Gaza/Israel isn't a normal environment. Gaza is, what, 300 square miles of what must be one of the most comprehensively monitored territories in the world? With a mile-wide no go area and another mile-wide exclusion zone around the whole perimeter. Palestinians have been shot for throwing stones in those areas. A Palestinian girl was shot dead while playing on the beach, no?
You must forgive people for finding it almost unbelievable that 3,000 Hamas fighters could drive up to the border, with a bulldozer, no less, and presumably hundreds of pick-ups, undetected. Add in the warnings from Egypt. Yet, this is what happened.
You would imagine there will be a completely independent public enquiry into what happened after all this is over. The Americans, I am pretty sure, would demand one and I am also pretty sure the conclusion won't be "intelligence isn't some foolproof guarantee terrorists won't ever succeed".