In its early days the Walt Disney company was far from having the same means as they do today. Its creator who the company is named after had to find ways to save money. He thus found a rather nifty method and even the biggest Disney fans probably haven’t noticed it.
At the time, in the 20th century, creating unique animations required a lot of work without the technologies that exist today. It was also necessary to have many designers and illustrators, and therefore enough money to pay them. To avoid doing extra work they took animations from old Disney productions and reused them in their new feature films.
When comparing certain scenes you can see that the gestures of some characters are exactly the same, and it's rather amazing.