Firstly, I'm not going to say that all self-published books are terrible. They aren't. That being said, I have read a decent amount of self-published authors and have found a common theme in the bad ones.
In one word: editing.
Editors can be expensive. A good editor can charge up to $10 per manuscript page. For an unpublished author with a novel around 60,000 words, that's nearly $800 in editing charges. Of course, that's on the expensive side. Cheaper editors come in around $1.50 - $2 per manuscript page. As with most things, you get what you pay for.
Why do self-published authors need editors so desperately? Credibility. If you know you are reading a self-published work and find a single typo, you are more likely to disregard the entire book as sophomoric and unworthy of your time. Is that judgement unfair? Yes, but that's a discussion for another time.
Also, editors do so much more than just turn a giggling stomach into a jiggling one (one of my actual edits from For We Are Many). I'm currently reading a self-published and entirely unsuccessful sci-fi novel. Hidden beneath all of the formatting errors and other issues lies a great story. Trying to slog through the novel is turning out to be a real chore - despite my enjoyment of the plot and characters. The paragraphs are often confusedly organized and I've never before read a novel with parenthetical sentences every other page. Those are all issues an editor could have cleaned up and the end result would have been a solid sci-fi I would recommend to friends.
In closing, if you are attempting to self-publish, get a budget. Plan on spending about $500 right out of the gate to make your story into a masterpiece. Set $150 aside for a cover illustration, $200 for editing and proofreading, and $150 for marketing and ordering paperbacks. That may seem like a massive amount of money, but you don't see investors sinking 20 bucks into the stock market in hopes of making millions.