Because as long as the Qassam remains an unguided missile with no target-seeking capabilities, it remains a weapon of terror and not of war. As such, it is no more than a satchel bomb that can be delivered from afar, with few guarantees that it will kill or injure anything but civilians. The last time I looked, deliberately attacking a country's civilians was an act of terror according to international law. That's why not.
U.S. Democratic Senator Merkley airs support for Iran nuclear deal (Reuters)
from the article: A state that abducted itself