Because sometimes you make assumptions that are seemingly correct but eventually found to be wrong or based on flawed inputs from sources beyond your control.

Any system that ultimately relies on "engineers need to always do the right thing" is a flawed, brittle, ineffectual system. Because even the best engineers will make a mistake somewhere, and because you can't exclusively hire "the best" engineers.

Let's spend our time figuring out how to recover from mistakes rather than trying to pretend they'll never happen.