Because the systems are so complex and capable of emergent behavior that you need a human in the loop to truly interpret behavior and impact. Just because an alert is going off doesn't mean that the alert was written properly, or is measuring the correct thing, or the customer is interpreting its meaning correctly, etc.
Health probes are at the easiest side of software complexity spectrum. It has nothing to do with it and everything with managing reputational damage in shady way.
