A most excellent question, why is it all these questions seem to arise as if I asked them myself?
I am not sure I can fully agree with the most exalted GK, for all novels must tell us something of the writer, for one can only write 'what you know,' and of all topics, the author hopefully knows himself best. We are also humans writing human stories (whether the main characters are technically human or not) and as such, all fiction must tell us something of the human experience or what is the point of reading or writing? If we are reading a fictional account and it reads like an autobiography with the main character being the author, then yes, that is a bad novel indeed, but I cannot write a story or a character without incorporating some part of my experience, my fears, my hopes, my joys and sorrows, yea, my very soul into what I do. For the root of all great art is buried deep in a human soul and without baring a little of that soul, we cannot hope to touch the world in any meaningful way. What we know about being human, and thus about writing believable stories, comes from our own experience and hence is translated into our writing, though hopefully not in an exact copy! In a good story, you will read a little bit of who and what the author is, but more importantly, you will learn some universal truth of what it is to be human.