To put it simply, if the product is useless the market for such a product fails quickly as anyone purchasing it soon realise it's a waste of money, .
This just isn't the case. People are marketed into believing things and hear what they want to hear and want to believe their purchase and judgement was valid and justified. Also there is a great propensity among the audiophile community to grossly exaggerate the differences that are real into a totally inappropriate level of significance.