A publisher is taking a gamble that your book will sell. They pay your advance. They edit and polish your prose. They pay for all the typesetting and design. They pay for printing the book.
Some publishers even help with marketing and advertising.
With all that money on the line, it makes complete sense for the publisher to be selective in what they publish, and to get the lion's share of the revenue. That's very little money for a lot of effort. No wonder most authors don't get anything more than their advance. The ebook model is completely different. You still have a publisher.