Most youtube videos I encounter have caveats saying stuff like “I DON’T OWN THIS VIDEO, I JUST FOUND IT ON THE INTERNET!” or “No copyrights intended, I’m not the owner of this song” etc. Why do people bother with these notices? They’re hardly going to protect them when the RIAA or MPAA et al turn up. Occasionally, you get people saying that “sharing videos is fair use” so they have some vague grasp of the law, and then very occasionally you’ll have a disclaimer like “This video is protected as a fair use under US copyright code gamma alpha beta 1945 that allows for parodies” or something to that effect.
I think it’s because people who don’t know much about the law get trademark and copyright law mixed up. In trademark law, you can often get by with such a disclaimer, as the problem is not using the trademark, but tricking people into thinking the trademark owners are sponsoring you.
It could also come from how you are taught about plagiarism in school. It isn’t plagiarism if you cite your sources. And the idea that plagiarism is a moral, not legal, concept is lost on a lot of people.