My two cents worth: Put yourself in "The Wayback Machine". You are Walt Disney, it is the 1930's and 1940's and 1950's and you are developing ideas for animated and live-action feature films, even theme park attractions and TV shows . Where can you acquire (ooooh....horrible word) familiar, popular or at least 'known' characters and stories for family audiences from so you can make successful product? Well, books are far and away the source of those things in a popular form back then, so you do it. Not just public domain fairy tales like "Snow White" but recent works like "Bambi" and "Dumbo", "Old Yeller", "20,000 Leagues Under the Sea", "Zorro", (I could go on and on) and even a franchise of sorts with "Mary Poppins" that has multiple books and a lot of characters in it. If you are Walt, you really don't have anywhere else to acquire stuff from. Your company is pretty much inventing family entertainment as you go, there aren't any 'franchises' as we know them today in film for families so you do a 'back in the day' version of what the company is doing today- buy the stories, characters and rework them in most cases into 'the Disney version' from books. Again...there weren't any franchises to buy in films, so we don't know if he would or wouldn't have bought them, given the chance. He DID buy other peoples work and use it. That's obvious, traditional and fine.
The company still creates stuff that they didn't 'franchise purchase' and release it to less success financial results than the biggies like Star Wars and Marvel. Some of my favorite Disney films of the past few years were "The Odd Life of Timothy Green" and "Saving Mr. Banks". Very original, very modest box office and the crazy original "Tomorrowland" which bombed badly. It's fine to not like that the company is purchasing franchises and just releasing their 'bought' output but it's unfair to say the company doesn't still make new stuff and it's inaccurate to say that the company used to be different in how they 'got' ideas. The scale and mediums have changed...not the process.