I was about to say the amount of time involved-- with a song, you'll know in a few seconds whether you like it or not, and figuring that out about a book takes longer. But then I realized that isn't true. If someone can't write, you know it within a few sentences.
But at least with indie musicians, you know that they have spent SOME time working on their material. They actually have learned to play instruments (somewhat) and write lyrics, and work together as a band. They are likely to have played in front of SOME number of people before, and seen their reactions.
With writing, all you need is a writing medium (word processor, type writer, notepad) and the conviction that you know how to write well and tell a compelling story. Most of the people who think that are wrong. And the more mentally imbalanced you might be, the more false certainty of of one's own skills you are likely to have.
The majority of PUBLISHED writers don't actually write very well (see Law, Sturgeon's) so what are the chances that someone who is incapable of getting a book through a publishing house is actually worth reading? Maybe 1 percent of the time, it will fail muster because the publisher/editor/"slushpile diver" can't recognize the hidden brilliance of the writing. The other 99 percent of the time, it is because it is crap. (Sturgeon was an optimist.)
http://en.wikipedia.org/wiki/Sturgeon%27s_Law