Basically, because I absolutely must know where I'm going before I write the journey. The plot might jump around a bit (if there is a plot, which is...sadly infrequent) but I always head into any story knowing exactly where I want my characters to end up both physically and in terms of any emotinal/relationship changes they may go through. And having the ending written means that I feel less like I'm flailing around without a compass when the middle section is being problematic, which it always does.

If the fic is more abstract, I might wait a bit and let the ending emerge as a way of tying up threads of reference or metaphor, but it always makes itself known pretty early in the writing process.