Because that's the question, isn't it? Sure, you can say "It's entertaining" - but if you are telling the exact same story that anyone could walk pass and write, why are you bothering? It's not just with fiction either - "story" here is any narrative form. What is it that you are bringing to the table?
I've seen more "I don't know"s over the years than I personally feel comfortable with. It's your writing. Kick the tires. Look under the hood. See where the magic pixies go in and out of the tubes that keeps that engine running.
Looking back, one of the saddest moments in my life was realizing that I could no longer answer the fundamental questions of - why am I writining? What can I do that isn't a reflection of someone better, but actually morphs the work into something uniquely me? Why am I passionate about writing?
Those things are important, fundamentally important to your existence as a writer. You lose them, you lose your sense of self. Or, maybe that was just me...
I know there are plenty of people who "just write for fun" or have only vaguely entertained the idea of calling themselves a writer. Maybe they don't even talk much about it in their real lives, because they have gotten the impression that it's a hobby or just something to kill the time.
I have a secret for you though. If you didn't take it at least somewhat seriously, if you weren't a little bit in love with the words that come out from your fingertips as they slide across the keyboard, you wouldn't be here right now. You most definitely wouldn't be in Week 24 and prepared to go as long as you can in this crazy thing we call Idol. intr
For all of the worry and concern that you might not be good enough - you are here. For all of the needing encouragement, you believe enough in yourself to hope that others see it as well.
Hopefully, at some point, you'll realize that and figure out what about all of this invigorates you and makes you so passionate about sharing your work with others, and what you are hoping they take away from that experience.
(and yes, a lot of this has to do with my own thought process following a conversation with an old friend, and thinking about how passionate I used to be about writing and about how to get that back... but never you mind that, we're talking about you here! ;P )