I think plagiarism is a form of theft. It's a way to make other people think you created something and did a certain amount of work when someone else did.
It shows contempt for real learning and hard work.
In some cultures, like Ancient Greece, authors paid homage to their predecessors by using the same plot lines or characters. However, they didn't plagiarize as they always added an new component to the work. They didn't pretend that the story was 100% original