The publisher would rather make a new version of some proven concept. A sequel to a successful game is less risky than a totally new game concept. Or a game base on Star Wars is easier to market than a game based on some new characters no one has ever heard of. To make matters worse, the publisher receives dozens of game designs from dozens of game developers every month. It is very difficult for a developer to get noticed by a publisher and make his game design stand out compared to all the other options the publisher has.
Publisher has all the control There are hundreds of games released each year.
Not very many are profitable, and a huge percentage of them are never even finished. Games are often canceled halfway through. So, a publisher is taking a big financial risk when they agree to publish a game. In exchange for taking that risk, they take most of the profits. And to help protect their investment, they take most of the control of the creative design process.