Because that's not what this Forum is for
There's nothing stopping you, or any other member of this website for that matter, from helping people write their stories. In fact, some of the members of Worldbuilding do help people write stories - just not on World Building itself.
World Building, when you get strict about it, is a simple question-answer forum which seeks to fulfill a single highly-specialized niche - establishing the setting of a story. Fundamentally, any story can be broken down into several discrete components, and one of those components is the setting of the story. And, furthermore, we specialize in a specific kind of building - that is, the how, not the why.
See, a good story has meaning to it, and symbolism, and all those things which make a reader identify with the story and enjoy reading the tale that's been spun for them. That is, the why. That's not what we care about. We care about the how. And it's this laserlike focus on the how which lets us function cleanly. Art, you see, is a matter of interpretation. Yesterday's masterpiece is today's dishrag and today's dishrag is tomorrow's masterpiece. Or, to put it in WBSE terms - it's too opinion based. But what isn't opinion based is science and reasoning - and those too broad categories are what you can fundamentally fit every problem here into. Either it's a question about how the world works - 'science', it's a question as to how to apply a fictional concept within a set of rules - 'reasoning', or occasionally it's both.
This allows the site to function cleanly, efficiently, and to serve everyone who wants to use it. And that is, after all, the goal. The goal of WBSE is to solve the how in World Building, and if we allow for additional features, we jeopardize that clean ability to solve problems.
Not to mention that there exists what you're looking for already everywhere else. There's a literary stack exchange which deals with more story-oriented questions, like character development and so on. Not to mention that there are numerous critique groups scattered throughout the internet which will be happy to welcome new writers. Given that we wouldn't actually be providing anything new by expanding the website and we'd be sacrificing the unique thing we have, it's just not worth it.