Why? Because, as I explained in Onward Muslim Soldiers, they object on Islamic grounds to the very existence of a state governed by non-Muslims on what they consider to be Islamic land. They will not rest until that state is obliterated, or they are. From WND:
Despite Israel’s decision to evacuate all Jewish communities and military positions in the [Gaza] area, Hamas leader Dr. Mahmoud al-Zahar says the “resistance” and war of “liberation” will go on.
“We do not trust the Israelis and we do not trust that the Israelis are going to withdraw from Gaza while they are speaking of controlling the sea and the air,” he said. “Until the occupation completely ends, our resistance will continue.”