I'm not a published writer, so my knowledge/opinion is likely a lot less informed than the rest of you, but I think this is a large part of it.

For years, authors have been told that self publishing has been the way to go if you can't 'cut it' in the 'real' publishing business. I mean it's been called (and may still be) Vanity Press; in two words, it relegates the author's work to an affectation rather than literature.

Jenny (below this comment) may have the right of it too. I have no doubt there are many motivations that go into this.