The problem with self-publication is that your book might as well not exist[0] unless you already have a significant following (in which case you could easily secure a publisher). This isn't an issue unique to the digital age, there's a term specifically for publishers who will just publish anything: vanity presses (though the catch is that you have to pay them usually, hence the "vanity" part). Publishers provide their name, marketing, and get your books into stores which is pretty important. (I write Wikipedia articles on manga and you can easily tell the North American publishers apart by how hard they get the manga scene to write about and review—via advance copies—their books, especially with regards to press releases and staff interviews. If a manga didn't have this, it became really hard to justify a Wikipedia article for it on the basis of Notability) Also I bet for statistics purposes, only books registered with the Library of Congress are counted (how else would you find all those self-published works?).

There are services like Lulu for free self-publication,[1] but they don't carry the same "legitimacy" or reach of publishers. I think the best analogue to online self-publishing would be the zine: they were easy to reproduce (via photocopying) and distribute (at events or via post). However, none of them really had a long-lasting legacy[2] and anything successful eventually legitimizes itself as a periodical or magazine and becomes established. Thus, I think self-publishing doesn't really change much for the individual, but makes it much easier for groups[3] to gain traction. Overall, being self-published on the internet just increases your accessibility, but we should be careful confusing it with traditional publishing or counting it in statistics because a lot of it is just noise. (bringing up Wikipedia again, there are tons of "books" on Google Books that are actually just random compilations of Wikipedia articles)

Anyway just some of my random thoughts, hope I didn't digress too much.

[0]: Overused thought experiment: "If a tree falls in a forest and no one is around to hear it, does it make a sound?"

[1]: https://www.lulu.com/

[2]: An exception like the Phrack ezine might be of interest to the HN crowd. (https://en.wikipedia.org/wiki/Phrack)

[3]: Here's where the print-web distinction breaks down. A ton of blogs and amateur news websites have evolved and became taken seriously. Just because they're not published in a book format, doesn't mean they're distinct in my opinion.