Because they don’t really believe that these people are terrorists. They know that if they actively starved Gaza, the people there would fight back hard to save their lives and the world would be so appalled that they would actively try to prevent the starvation. Mass starvation in Gaza would have other nations in the middle east actively step in to provide aid, actually useful aid, that would benefit the Palestinians, not the crumbs they are given. With actually useful aid, they could mount a much more solid defence against Israeli aggression. So this used as a means of slowing down the slaughter of the Palestinians, making it run in slow motion, the “frog in a pot of boiling water” thing, rather than it just being quick and having actually useful humanitarian aid arrive there.