Gaza has an elaborate underground tunnel system in which Hamas can move around, and Israel's military has no visibility. That is part of the reason the war is so cruel. Urban war is hell; Urban underground war is doubly hell. Any basic military strategist will recognize that without removing the tall structures above, till visibility can be achieved, war won't be won. How can you move people to humanitarian zone? It is not like Hamas wears uniform and fights. They will mix with women and children and pick on Israeli forces. This was the same problem U.S. forces faced in Iraq urban war. They were constantly getting killed by following traditional rules of engagement when a kid would plant an IED. They didn't even have tunnels. The situation was a bit different in Afghanistan where the Al-Queda and Taliban could hide in the caves. But, Afghanistan being mostly empty, U.S. troops could use high explosives to neutralize the cave dwellers. That is not possible in a dense urban place like Gaza. It is easy to criticize Israeli war tactics, but nobody has offered a much superior option.
See the challenges U.S. faced in urban warfare, without tunnels and much less density:
en.wikipedia.org
en.wikipedia.org