Warning: if by any chance you haven't seen the 1939 film The Wizard of Oz or read the 1900 book The Wonderful Wizard of Oz, this post and the linked article contains spoilers.
When the producers of The Wizard of Oz adapted the story for the big screen, they changed a lot of details and plot points to make them manageable for the production crew, or make them look better in color. The complex and sometimes overly scary story had to be simplified to fit into a feature film format and allow time for the songs. The one big change that was totally unnecessary and confounds today's filmmakers was the decision to make it all a dream. They supposedly did this to make the fantasy more believable to audiences. But that also cut off the possibility of the more modern practice of making sequel after sequel, which would have been easy considering L. Frank Baum had written a slew of books about Oz.
There were many changes made between the book and the film, ten of which you can read about at Mental Floss.
(Image credit: MGM)
And they did make a sequel, Return to Oz, which is actually pretty good, if maybe too scary for really young children.