>> Frequency response and total harmonic distortion aren't measured in these cases because they're useful or relevant. They're measured because they're easy to measure.

We've evolved our senses around the signal components the pay off the best: the "easy" ones. Thus it isn't surprising that "subjective quality is mostly correlated with linear (spectral) attributes instead of non-linear (distortion) metrics," a claim supported by peer reviewed papers that have vastly greater credibility than you and your anecdotal vitriol.