They don't. If I make a change that prevents a death you will never know about it. If I make that change in response to a death you will hear about it, thus biasing you into thinking I don't care until there are no deaths. In reality though sometimes you need an actual situation to happen before you can imagine that failure most. Trying to guess how your widget could kill something before their are any deaths is hard. (in some cases you can look at existing widgets, but there is always something new)