They only have limited resources and were using those to defend areas where Hamas were actively attacking. If they’d put all their troops to defend mostly empty desert then the weakspot would have been somewhere else and they’d have targeted their attack at that weakspot instead.

War isn’t easy, certainly not against well funded terrorist organisations. I’m sure the IDF knew of hundreds of potential attacks and plans that Hamas wanted to carry out, knowing what they’ll actually do and when is an impossible feat - I bet they’re weren’t many in Hamas itself that had any idea what was going to happen, even Iran who trains and funds them appeared surprised.