Why do we desire authenticity?
Why do we want to be the cause of our own happiness rather than, say, medication? Why do we want to know that the jazz musician is truly improvising her solo rather than playing some pre-composed part crafted to sound improvised? Why is it so important to us that we experience the real world, and not a utopian virtual reality fed to us by machines?
I don't have an answer at all, except to say what you already know: in our society authenticity is usually valued, and its opposite (which might be deception, or B.S., or being a phony) is usually disparaged. Why do we hold some values on high and reject others? Authenticity, as a value, is related to truth telling. Yet why is truth better than falsity? My buddy Nietzsche would say that it may not be. We are free to take fresh looks at long established values, perhaps to uncover the unsavory, hidden dark sides of our values - even values such as 'truth.' I really liked your question and have enjoyed being puzzled by it!