Because you neither paid, nor credited, nor even acknowledged the artists whose work you're now implicitly using. This is pretty simple. If you go out and buy a bunch of art books and you look through those to get ideas, the people who made the art books get the money one-for-one for every studio that ever buys those books. If you go to someone's portfolio page on the internet and get some ideas, you now know who that person is and maybe you really like what they did. Maybe you might hire them to do concept art for your game someday. If you ask a generative AI to generate ideas for you, you pay Google, OpenAI, Midjourney, etc., and that's it. The artists who made everything that goes into it get nothing. They do not get money. They do not get exposure. They do not even get the bare minimum acknowledgement from you that they even existed. I am glad players are angry about generative AI. I hope they get even more angry. Artists that contribute art to a creative process deserve better treatment than this. If you like generative AI as a tool, advocate for a properly licensed, properly attributed version of it first. If instead you choose to use generative AI and ignore the ethical issues, you more than deserve the backlash.