It also depends on the skills of the writer. Believe it or not, not every author out there has amazing marketing savvy and tech experience to self epublish. Sure, I guess you can always hire people to do that for you, but then you also need the funds. (it should also be noted that those who succeed in self publishing do so because they put A LOT of work into it. Case in point the very famous Amanda Hocking, who discusses your initial question
here - why traditional pubbing? - and talks about how much work she put into her self publishing
here)
There are advantages to every form of publishing, and the key is knowing what form suits you as a writer best.
But since you won't often hear the advantages of commercial publishing extolled these days as most people want to put it down in order to build up self publishing, let me share a few pluses:
- an advance (yes you might make a lot self publishing, you might also not, an advance is money an author gets upfront and is always theirs unless they breach contract)
- editing, design, layout, cover art, and ebook formatting for different devices - for free!
- marketing - it might not be posters in the subway, but they at least do the minimum which is including one's book in their catalogue that goes to bookstores, libraries, etc., they take your work to major trade shows, send out review copies - and this is the minimum, most authors I know have had their publisher also arrange interviews with various media, advertise on popular forums that suit the genre of book, and in my case sent me in person to various book festivals, arranged events and panels for me to be a part of.
And while yes the author can always do more marketing himself, the key thing to all of the above is that the author doesn't have to do or pay for any of that stuff. So they are already steps ahead of where someone doing it all on their own would be.
Now there are authors out there who are good at all the above, and would rather do it all themselves and I say that's awesome. But me? I like having someone helping me, for free, and also I like getting paid.
However, all this said, I do have one book that I think would suit self publishing and am seriously contemplating going that route with it, so I'm not against it. I'm a fan of using all possible avenues to help get my work out there as it is suited to each individual book.