They have enough boats to evacuate the boat before it would sink. Well, that is the useual idea. Boats tend to follow shipping lanes and they rarely are that far from rescue. If the boat sinks like a rock you won't have time to get to lifeboats. If it starts sinking slowly you don't need to work that hard to putter people over to a rescue vessel in a timely fashion. It just doesn't make intuitive sense to us because the sea is a hostile environment in which we really don't belong.