Why write a story before making a movie?

I have seen and heard that producers and studios prefer to invest in a movie if the story is based on a published book. Why? To avoid or minimize the copyright wars. So if you want to make a book first then consider self publishing. But then it writing a whole story and to have it published in itself is a whole new ball game. Anyway to publish your story yourself here are some pointers:

• http://www.greenleafbookgroup.com/node/3774
• http://www.blurb.com/self-publishing
• Amazon's https://www.createspace.com/
• http://www.48hrbooks.com/Self-publishing

Read http://www.wikihow.com/Self-Publish-a-Book too; it has some nice tips.
 

Do you know of any other ways to avoid or minimize copyright wars and to entyce producers/studios?