I blame it on traditional publishing companies and old habits. My mom was a book editor, and the very best authors they published would sell 10,000 books. Their cut was something like $2/book, they'd pull in $20k.
So when I decided to write a book on growth hacking (https://www.secretsaucenow.com), I decided to self-publish. This was more difficult, I had to find an editor and pay them, had to market it myself, etc. Luckily I'm pretty good at the marketing part (hence the book) and I found old contacts of my mom's to edit, but we launched on Kickstarter, so I figured if it flopped I would know in advance.
We sold $66,000 on Kickstarter, then went on to sell another $44,000 on Indiegogo while we finished it up.
Since then we've sold around $20,000 on gumroad. In total it's something like 2,000 copies (though we charge a lot more per copy than most book publishers do, and that keeps us out of Amazon, etc.)
If I had a publishing deal I would make about $5,000 from that. But because I self-published I took home $130,000 minus costs and fees, so we'll conservatively say $100,000.
Now I sit back and check gumroad a couple times a day; I average $800 in sales per day, and almost make more from the book than I do from my (well-paying) full-time job. If I can keep this rate up I will replace my salary (or, rather, double it)