Occasionally, I see this question, and I thought I'd give it an answer. The short answer is, the same reason you don't go into work from 9-5 every day and refuse a paycheck. People who work have an expectation of pay for their work. It seems reasonable to most of us.
The second way I can answer this question is this way: I put time into each book, and before I start any book, I put a lot of time into learning the craft.
|Started in 2005, finished in 2014|
Learning the craft of writing.
This is no less difficult an endeavor that learning the violin well enough to get 17th chair in a symphony orchestra, or to play baseball well enough to be on a AAA farm team. It takes years of work, practice, and study. It takes coaching, or classes, which you pay for.
Malcolm Gladwell talks about the hours it takes to reach excellence. Writers talk about the 1,000,000 words they had to write to achieve a professional standard. Interestingly, these are the same number! A very hard working writer with no day job, no kids, no sick parent to take care of could accomplish that in three years. Most writers take more like ten years.
During those 3-10 years, you might sell a story here and there, or you might even self-publish and sell 500 copies of a book (which isn't bad for a first book!), but you probably won't gross much over $1000 in any given year, and that's for about 1,000 hours of work. A dollar an hour if you have no expenses.
Also, you sacrifice in your life. Other people come home from work, watch TV and play games and go on vacation. You probably don't. You use all your spare time to write, often when you're exhausted from the day job. (I'm assuming there's no inheritance or trust fund or rich spouse supporting you). Nor do writer's homes clean themselves and meals make themselves. Life is work, and then you work at home and self-maintenance tasks, and then more work is required to write. It's a sacrifice.
The costs of self publishing.
To put out a good product, you're looking at something like:
$400 for a pro cover
$300 for pro proofreading
$100 for interior formatting
$ ??? for advertising. $0 to $50,000/year, as some friends of mine have spent (they might make back $150,000, so they feel it's worth it.)
The time it takes to research and write a book.
Let's say you've done your ten-year, million-word writing apprenticeship. And let's say you have saved up $1000 to self-publish your first pro-level novel. How much time does that one take?
Probably six months to a year, or 500 to 1000 hours for that first good one. That includes: research, thinking, outlining, drafting, revising, line editing, proofreading, and running pieces of it past experts to make sure it's accurate. As time goes on, you can whittle down the hours it takes, but it still takes time. I have a number of ebooks that have made me $10,000 or so. If I spent 1000 hours on one, I'm making $10US per hour. If I spent 500 hours on it, I make $20US per hour. It's not a great salary.
Here's the crazy thing.
For the last ten years, I've been one of the lucky ones, making enough money on 5-35 titles to pay my bills and call myself a full-time writer. I know how little I make when I'm the 2000th best selling author at Amazon. It's like entry-level money for someone with a university degree. Novelists generally do not make much money, and in trade publishing (the books you can find at your library), it's even worse money per sale.
I hear people complain that a five-book series that took an indie author two years of full-time labor to write is overpriced at $25 if bought individually (though most indies run sales on first books in series and often bundle 3/$10). People spend $25 on a minor league baseball game ticket or $25 on a hotdog and pop at a major league event. They spend that for popcorn and a matinee movie that lasts 90 minutes. They spend more than that every month for their streaming, and probably that much weekly on their coffee out. They spend four times as much on their internet service and at least three times as much on their cell phone bill every month. A single trade published hardcover would cost more.
When it all comes down to it, and even not considering all those details, books cost money because we live in a society where people are charged for food and rent, and those people expect to be paid for their work in order to eat and have a roof. It's hard to write when you haven't eaten for a week, and it's impossible to get the computer charged when you live out of a shopping cart on some cold city street.