Apart from Michael Eisner's contributions to successfully rebuilding Disney's film studios (animated and live action), one of his more commendable traits initially was a willingness to work together with outside companies or even rivals. The most prominent example was Who Framed Roger Rabbit. Disney worked with Warner Bros, Paramount and other companies to bring all of their classics together in a single film that paid tribute to animation history. It was also a monumental triumph, with universal acclaim. This was during Eisner's heyday, before going insane with power and burning all the bridges with affiliates (and helping to ruin his own studio and creating new enemies).
Great Movie Ride was another example of Disney's willingness to work with other companies at the time. And no they didn't have to do it. Disney has tons of classic beloved and popular properties of their own that they could have filled out the ride with instead if they wanted. Not even including classic animated films like Fantasia. Mary Poppins, Bedknobs and Broomsticks, Absent Minded Professor, 20k Leagues Under the Sea, Davy Crockett, Treasure Island, Old Yeller, The ****gy Dog (Really? THAT is censored?), Darby O'Gill, Swiss Family Robinson, Babes in Toyland, The Love Bug etc.