Combinatorial complextity. It's impossible to foresee every possible combination of failures. There are checklists for the ones that can be foreseen (and these

Combinatorial complextity. It's impossible to foresee every possible combination of failures. There are checklists for the ones that can be foreseen (and these have often been written in the blood of those who discovered them) and maybe in theory most of those could be automated. But they cannot be exhaustive. So when all else fails, you rely on human intelligence guided by experience and training as the last line of defense. It doesn't always work, but until we have an AI that performs better, it's the best option we have.