Because it is the harvesting process which is the issue. Once you are past that process it may as well be used.
I dont think they understand that in order to get a live organ someone has to die as a result of removal of the organ.
@ChaimMoskowitz deems death to occur when there is no brain activity. We, as Orthodox Jews do not hold that belief. Therefore, someone on life support, even with no brain activity is deemed living and removal of their organs is tantamount to killing that person.
It is that simple.
So the real difference in opinion between CM and us relates to the pulling the plug and when does death occur questions.