Because the criteria will inevitably be biased towards technology and techniques that favor Google, Chrome, AMP, etc. If you trust Google to be an unbiased arbiter of objective measurements of website efficiency, you are giving them a lot more credit than I would at this point.

In the United States, the role of a central bank is to manage two measures: inflation and unemployment.

The US Federal Reserve Bank does not measure inflation or unemployment. Rather, the US Department of Labour, an independent organisation (and a branch of government rather than a freestanding entity) computes both measures.

This is the principle of division of responsibility and measurement or judgement. You cut, I choose.

Google are both measuring, and rewarding, website performance, as well as designing and distributing the principle tools that benefit by both choices. That's an extreme locus of power. And, history shows, generally a Bad Idea which Ends Poorly.

Because Google is the decider of what constitutes "slow"? You have a single organization deciding what the threshold is between slow and fast. And, they also have a Google-approved solution (amp) that I'm sure they will tie into the recommended actions for "slow" pages.

I'm not saying it's a walled garden (that's a more extreme situation on the continuum between open and closed), I was just using the phrase to illustrate my point that the end-user experience isn't the only factor to consider.