This is because of the very large possibility that their perception is compromised by various forms of bias and placebo. All of these psychological phenomena are well documented. Generally speaking, based on scientific research in this field, it is more highly probable that some form of bias is responsible for the difference being heard, rather than an actual, real objective change that can be measured with apparatus far more sensitive than human hearing. Its just a fact of life - our perception can be fooled extremely easily, particularly when we [i]want[/i] to hear an improvement. This is not to say that many audiophile myths arent grounded in reality. It just means that we need to be very careful and approach every audiophile belief with an appropriate amount of scepticism and scientific scrutiny.