Quite the contrary - they have all the power, they are just choosing not to use it in this case. If the court ordered a ceasefire all weapon shipments to Israel would have to stop the same day.
What does "have to" mean in this context? If the US were to sell another batch of weapons would other countries try to shoot the plane out of the air? Would they try to unilaterally sanction 30% of the world economy?
Those bodies have zero power and countries that want to massacre will kill no matter what.