The whole book is a series of reasons for not writing, people who didn't write, people who destroyed their writing, people who didn't finish, etc. Since I myself am the writer who doesn't, it's an attractive topic.
My father wrote a novel called Tenth which took as its theme the fact that great composers don't finish their tenth symphonies, and our protagonist takes on the task of finishing one. A nice touch in Tenth is that the composer in question is Thomas Mann's fictional Adrian Leverkuhn.
I don't know why I can't write. I haven't since about 1995, really. It's not that I think the world is deprived of some wonderful thing I have inside me. It's more that I feel constipated and grumpy about it.
There are no mute, inglorious Miltons, save in the hallucinations of poets. The one sound test of a Milton is that he functions as a Milton.— H.L. Mencken