The expectation to make money from artificially restricting an abundant resource. While copyright is a way to create funding, it also massively harms society by restricting future creators from being able to freely reuse previous works. Modern ways to deal with this are patronage, government funding, foundations (e.g. NLNet) and crowdfunding.
Also, plagiarism has nothing to do with copyright. It has to do with attribution. This is easily proven: you can plagiarise Beethoven's music even though it's public domain.
https://questioncopyright.org/minute-memes-credit-is-due