Yeah, that's the main reason because it's not your own work which can be copyrighted at all, technically you or any individual spends approximately 2 minutes to create a prompt and then expect to win a competition, if it worked like that, every GFX person would be a millionaire..
I don't know about the others but I personally create my own assets or paid for them on forums related to graphic design, those were back in 2008 until now and can prove out my legitimacy as I was called many times for using AI which I despise, however, the only thing I use is the render which is cut out by someone else and even if neither of us owns any rights on said render, we are not trying to sell it or profit with it in any possible way which is why AI artwork and the artwork I do isn't the same.