Maybe it's easy for Google to validate it this way? So AMP has a lot of limitations and if you're AMP compatible Google knows you're meeting certain base guarantees. If Google have to rate arbitrary sites and advise how to restrict them maybe it's harder to rate the performance guarantees.

I agree your approach would have a net benefit as well but Google's AMP approach of making you build your site with performance in mind from the ground up might be more effective than nudging existing poorly performing sites towards being more performance minded.