Because the kind of people who start software companies don't participate in book clubs.
Same answer for "Why isn't there a startup for knitters?", "Why isn't there a startup for school supply purchasing agents?", and "Why isn't there a software company which makes educational bingo cards?" (OK, last one perhaps not a great example.)
P.S. Somebody explain to me why more people haven't clued in that a) adult women have money and b) nobody else is trying to get it with software. Oh no, lets take the 473rd attempt at showing a 17 year old unemployed World of Warcraft player banner advertisements instead.
That doesn't matter. The average American only plays poker maybe once a month, but Party Poker still makes a billion. Startups don't need to target average Joe, they just need a big enough audience to make money, and there are a lot of bookclubs out there.
