Because history is littered with instances where overwhelming military superiority was used to effectively eliminate a guerrila force. They'll kill some Hamas fighters, but for every civilian they kill in the process they'll recruit 3 more. Cycle continues.
As long as Israel illegally occupies and oppresses the Palestinians, they will fight back. The whole sorry mess is bad people doing bad things on all sides, but carrying on doing the same thing over, and over again.....yeah sure *this time* Isreal are going to 'win' and the bloodshed will be over