But is that really true? Such classic works as the Narnia books, and the movie Casablanca (named second best movie of all time by the American Film Institute in 1998) were all written with very specific agenda message in mind. C.S. Lewis was writing a metaphor about Christianity - Murray Burnett and Joan Alison, the authors of the original screenplay that became Casasblanca wanted to write a story to convince America to join World War II.
The books Black Beauty and Uncle Tom's Cabin don't usually make people's top ten lists, but they had a huge impact on their world, using narrative to change people's opinions. Uncle Tom's Cabin had such an impact that upon meeting Harriet Beecher Stowe during the height of the Civil War, Abraham Lincoln supposedly said: "So you're the little woman who wrote the book that made this great war!" Stowe could have used Western Union for her message, but I don't think she would have gotten the message through as effectively.
So, in short, of course there are many lousy stories out there trying to preach a message, but there are plenty of lousy stories out there that aren't preaching a message. A great story can have a message.