Because, as is demonstrable, the question of whether objective moral values and duties exist is an incredibly important question, whether you realize it or not. Regardless, although I think that objective moral values and duties clearly do exist (for example, we have a duty to love a child rather than to abuse and murder it, or that the crimes of the Nazi regime were objectively wrong, regardless if even a majority of society were to agree or were to be brainwashed into believing that they were right -- etc), we must come to understand and help people realize what it means if morality is subjective. It may very well be the case that morality is nothing but an illusion -- we must simply understand what it would mean if this is true.