Because, it seems to me, a good writer who writes quickly will write a better book most likely than the slower writer who isn't as good a writer.
I've been trying to redo the opening of my ms for 2 weeks now. Last night I spent over an hour on the first sentence yet again. I doubt it's all that great given how much time I worked on it. People who want to write should just write and not worry about such things unless they're on deadline. And no editor, agent, or reader (unless they've been breathlessly waiting for the ms) will know or care how long it took to write a book. All they'll care about is if it meets their definition of good.