I know a number of authors who self publish their work. There are any number of reasons - creative control of content and cover art, being able to put out new books whenever they want instead of having to wait months and months to have a book released, not being able to find an agent and/or publishing house they are happy with, the much larger royalty percentages, and the quicker payout of them.
I have to admit I've thought about going this route. The royalty percentages alone are enough to make me consider it. But I don't know anything about formatting a manuscript into the correct format for a self publishing platform and I don't know anything about designing cover art. I realize there are companies and/or individuals I can hire to do these things for me, but I hate the thought of paying for something where there is no guarantee I'll make my money back. Of course I could learn how to do it myself, but I don't know where I'd find the time to learn and practice with it enough so I'd feel comfortable putting it out into the marketplace.
One final reason is self published authors are much more likely to be the targets of plagiarism those those published by major publishing houses. Here's an article that details how prevalent it is and how hard it is to fight against. Plagiarism In The Age of Self Publishing