Why do newcomers often make plagiarism mistakes?
Because the society we're coming from has failed and the world is corrupt basically before any other reason. Nowadays, humanity don't want to work before earning, they're always looking for the shortest way possible which most times involves cheating to get there. This is the mindset of the typical human and how the real world operate so they bring that same mentality into the forum.
Ethically, you shouldn't need any rules to tell you is not ok coping someone else works without giving credit then passing it as yours, taking all the glory and reward for that content be it an article or graphic.
The society has normalize stealing and other immoral things like promoting scams surfer you're not been affected. We're now cool with influencers promoting projects that are shady like most of the hyped meme and Bitcoin clones which were only created to steal your bitcoin.