Have you talked with a lot of artists about it?
Labels are especially helpful now, when PR job is a thankless grunt work involving a lot of know-how. Personally, I've worked with a couple of labels and promotion groups, and the amount of effort label puts into promotion has tremendous impact on the outcome.
Basically, there's almost no listeners who actively look for new artists and music that is outside of view of some spot of spotlight — and to get into that spotlight, you have to work. I'm not even talking about publications of scale of Pitchfork or xlr8r now; much smaller blogs get flooded with review requests and demos.
But most importantly, the kind of activity you have to do to actively sell yourself on social media is completely psychologically repulsive to a lot of artists. I've talked to a lot of people about it, and most agree that while they could see themselves pushing someone else's creations, doing it for themselves too often pushes one into apathy, self-doubt and depression.
The awesome stories that you hear about self-made success suffer from survivorship bias. Labels are there for a reason, and they deserve every penny that they take.
Edit: sorry, I missed your link. I get ~80% of my releases; didn't realize you were talking about the big dinosaur labels here.