One of the issues is that although we can measure a heck of a lot about an amplifier, we can’t express those measurements in ways that are easy to digest and interpret for the average reader. We end up with data that is easy to misinterpret or take out of context. Part of the blame lies with consumers, who want to compare and contrast specs as a replacement for firsthand experience, and part lies with marketing departments that are being deliberately selective or unintentionally incomplete with what they publish.
THD is one of the classic examples of this. The THD as a percent often appears in marketing specifications, but it is not nearly so often accompanied by the all-important context. To make practical sense of a THD specification, you need to know what kind of signal was applied during measurement (frequency, level, single sine wave, multiples for IMD, program material, etc) and at what level the output was measured (voltage or wattage). Even when that information is provided, judging % THD across devices doesn’t give you a good comparative idea of the sound unless you know the harmonic makeup of the distortion spectrum (very rarely provided by manufacturers). All 0.X% THD measurements are not created equal in terms of your listening perception.
Power output is another example of specs that tell less than they should if we want to make a judgement on numbers alone. Like THD, power output requires context in the form of % THD at the measured level, the load used to measure (reactive load, resistive load, etc), and the signal used to measure. This information is not often given and so apples-to-apples comparisons that would allow someone to say with confidence that “these are equally neutral amplifiers” really is not possible. To make matters worse, most of these measurements are interactive, making expressing the results in a practical way more difficult.
While we hope manufacturers would include this kind of information in specs as we shop, it just does not happen. A big reason for that is the average person just doesn’t care. The responsibility falls to the niche press that reviews products and is able to measure them with a consistent method. But there’s a diminishing return on the amount of work it takes to measure and share all the data needed to give a clear picture of how something sounds. Even if the measurement work is done, it takes a certain amount of technical knowledge and experience to interpret it, so we end up with catch-all terms like ‘warm’ or ‘neutral’ to paint the picture. This audiophile language is as much a shorthand for all the measurements that aren’t being done as it is flowery jargon.
TL;DR It takes a huge amount of work to measure and express the findings if you want to capture all there is to say about how something sounds. Few are willing to do it, so we have words like ‘neutral’ that are part subjectivity and part experienced evaluation by trained ears. Even then, it’s not a perfect substitute for hearing something yourself.