Because the rank algorithms are subjective heuristics, not absolute metrics. All rank algorithms always have been. It started with the link metrics, then people

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 That's just not their role and it will be out of date before it's published.