Stats don't purport to ... they measure results ... the physical, mental - whatever - are the stuff that goes into getting results. If measurable results aren't there - then there is a deficiency in the other stuff (physical, mental, general character, body odor, et al.).
Players are expert on the inputs (mostly). I am sure Koosman was more liked than Seaver - Seaver's results were better, when the games mattered a lot and when they mattered less. That is mastery of the mental, physical whatever aspects.
The "stats don't measure" argument is a terrific strawman - because of course they don't.