If I write a book, in all honesty nobody is going to read it, even for free.
But if through some curse/miracle I become an author, then I have two choices:
1. I write maybe a book or two every few years while doing other jobs to earn money to live. And the books provide an occasional bonus, but not one I can rely on.
2. Somebody pays me to write, which means I can now write maybe four books a year as it's now my job. And for me to get paid to write, other people need to pay money to read my books.
As a reader, if I buy a real book, I can sell it. I can lend it to friend. I can eat or burn it if I so choose. Either way, it's mine to do what I like but once it's gone it's gone. If I want to give it away again and read it again I have to buy another one, thus paying the author again. And the printer, bookshop etc. My generosity it truly out of my own pocket and not at the expense of anyone else.
With PDF and the like, I can make as many copies as I like and still do what I like with every copy, at no cost to myself. I am of course ignoring copyright and courts in this example, but they only act after the event and the author still doesn't get any money after I'm sent down. If this is taken to the extreme, any author will only ever "sell" one copy.
Whoever can solve this problem to allow you have digital versions of books (with all the advantages of surviving being dropped in puddles or just plain worn out) with the simple ease of use of the genuine dead tree article and none of the customer unfriendly DRM / rental schemes we have now is going make a fortune.
Charlie Stross did a good series of articles [ see www.antipope.org - the domain is a typo and long-running joke and nothing to do with popes] on the real world of being an author, the effort involved and the whole process from idea to print and the full reasoning I've attempted to paraphrase from memory above. He also hates DRM, has free stories as well as info on the ones he'd really like you to buy - and I'd recommend you do too. Preferably from your local bookshop.
And if you think authors could live off voluntary donations and good karma, just look at all the people who wrote shareware or write free software today and how few of them can actually do it without being employed to do so - another very topical discussion in these august pages. I'm not a programmer either.