What's so scary about big heavy books?

Why is it that we feel daunted by the prospect of starting a really long book? We know it will take a long time to finish it, but why does this matter? Surely we don't read for the sake of the sense of achievement at finishing something, we read for the pleasure we get doing it. And most of the time, the more you get into a book the more pleasure you get; it's the first 50 pages that are often the hardest going. So if anything it takes less effort to read one 800-page novel than four of 200 pages each. Yet somehow, in front of the bookshelf, we feel scared of the commitment.