Because the rank algorithms are subjective heuristics, not absolute metrics. All rank algorithms always have been. It started with the link metrics, then people started gaming that. It's been a signal/noise war ever since.

It's also dangerous to ask for the exact criteria because they are ever changing. Google et al don't want to be prescriptive about what a good site is, they want to recognize what a good one is. You make a good one, they'll figure out how to recognize it.

They can't sit down and publish "The Definitive Guide to a Good Website". That's just not their role and it will be out of date before it's published.