This. The writing and editing parts are trivial in comparison with getting people to know your books exist. I can see the appeal of a publisher if you're writing in hopes of actually being read -- not because the publisher does any work for you, but because the fact that your book is associated with a publisher means that more people are willing to risk reading it. If you self-publish, and even if you paid to have your book edited, you're still going to have huge amounts of trouble attracting the first tiny bit of an audience unless you are writing some obscure non-fictional stuff that can be judged at a glance as useful or not, and maybe even then you'll still have trouble. I only have experience with self-publishing fiction, so I can't really say for sure on the latter.
That said, people severely underestimate the gatekeeper effect that these traditional publishers play, not just for books but for music as well. It's not that I think that publishers are actually _good_ arbiters of taste or quality, but I can't deny that people would prefer to take a chance on a crappy song that is getting radio play after being pushed by labels than they are to waste 5 minutes listening to random MP3s on some guy's website. Truthfully, it's hard to blame them. I'm a writer and I make electronica, and I still find myself hesitant to waste time on random stuff I find online, simply because of a few bad early experiences doing that. So I'm sure that I'm missing great content in the same way that some people would probably really enjoy the things I create but skip it rather than take the risk.
The short of it is, human nature is to blame here. People usually (and fairly rationally, I think) prefer the guaranteed payout in entertainment that they expect from a curated source, one that they are familiar with, to the real risk of wasting their time listening to or reading horribly flawed creations that they randomly stumble across online. The only way around this, from a creator's point of view, is to either delegate the marketing jobs to a publisher or to spend a lot of time doing it yourself. For the average writer/artist/programmer/musician/etc, I think that's something that is not really much fun when you'd rather just be making more stuff.