Here's why:
* They were designed specifically for the film: Costume designer Adrian designed the iconic costumes, drawing inspiration from the book but also creating unique looks for each character.
* They were made for the actors: The costumes were tailored to fit Judy Garland, Ray Bolger, Jack Haley, and Bert Lahr, and others who played the main roles.
However, while the costumes were created for the film, they are now part of museum collections and are sometimes displayed in exhibits, giving the impression that they were "found." For example:
* The Academy of Motion Picture Arts and Sciences: The Academy holds a collection of costumes from The Wizard of Oz.
* The Smithsonian National Museum of American History: The Smithsonian also has some costumes from the film.
So, while the costumes weren't "found," they have been preserved and are now part of historical collections.