Because culture is built on itself. Almost everything is built, remixed, rewritten at some level. A good chunk of the Disney classics are taken from older stories, fairy tales, etc that were by then public domain.

Borrowing/"stealing"/copying happens with movies: https://www.youtube.com/watch?v=Z-HuenDPZw0 (fast forward to 3:12 to see how much star wars borrowed from other pieces)

It happens with music (kirby had to remove this originally because of...copyright) https://www.youtube.com/watch?v=zmjHlkQYoOM