It's because perception and our senses are pretty much entirely subjective and perception from one sense is influenced by the others. Your sight, hearing, smell and taste can change depending on the conditions, sometimes quite quickly. I don't mean synesthesia.
Coupled with the fact one can never experience exactly what someone else experiences, meaning we can only describe something with words (which are themselves somewhat subjective) means one can describe a sound or feeling in a way that cannot be tested, validated, or confirmed. They may very well be genuinely trying to describe what they "hear", but it is difficult without referring to other sounds, just like trying to describe the colour blue to someone who was born blind.
For many it seems audio gear provides an "experience" which includes the knowledge they have spent alot of money on something they desire, so it must sound "better" (in whatever way they wish to describe that improviement). I'm sure to them, it really does sound "better" but that is both relative and subjective. What we percepieve is based on preconcieved ideas.
Ultimately the terms you described have no value other than for salesmen. I'm am not suggesting its all "lies", just that with no objective measure, or specific properties to measure objectively, its somewhat meaningless. Some properties are of course quantifiable - noise floor, bandwidth, THD etc..
Couple all this with an industry that - like most businesses - exists to make money, and there is a market for extremely expensive audio gear for those who have lots of money to spend, and want to spend a lot of audio gear. To justify that cost, manufacturers must use materials and technologies that aren't used on consumer gear - it doesn't actually matter if those materials/methods/techology are in any way beneficial to the quality of sound - but if you charge $50,000 for a 30W amplifier, why would it cost that much unless you're doing something "better". After all, people generally believe that, the more something costs, the "better" it must be.
Re: Placebo. Placebo effects (because its more than just one) are most strongly associated with subjective measures (fatigue, pain, anxiety sight, hearing etc..), and tends to be very weak for objective measures (actual medical outcomes, serum levels etc..). Given hearing is very subjective it's prone to strong placebo effects/biases.