Poems, articles, columns, and short stories are all creative commitments, sure. But even if they linger unfinished for a while, they are short relationships compared to 350 pages of manuscript.
With a book, you regularly re-evaluate your progress, your purpose, and your plans. You recommit again and again. Not unlike the work it takes to make a marriage work.
Many of my students weary of this. Is it ever done? they ask. When is enough, enough?