My guess to why there is no direct comparison is time. It would require 6 / 7 thousand miles of real road testing on comparable bikes, road conditions and the same riding style to get an accurate test. Not many organizations have time and budget for that.
I am thinking of lab testing for the objective criteria. There are several tire test labs in US and Germany such as Testing Division - Standards Testing Labs - Tire and Wheel Testing Experts
At least that would provide some basic quantitative data that is more beneficial than the subjective stuff we read/hear.