Why would deer continue to cross a river full of crocodiles even though some of them have been killed?
The reason for their behavior is that exactly the fact that even though some of them do indeed get killed, most of them survive -- what you have said.  The deer cross the river because that allows for the better continuation of the species rather than if they had stayed on the original side of the river. You could think of it this way: If there was a crowd of 5000 people crossing a field of grass and aliens were abducting them one by one, the actual chances of them abducting you are rather little.