They are fed with art the AI owner don't have the rights to in almost all cases. They scrape art sites an ladle it into the AI to be analyses so it can imitatebshapes and stiles. Without that "seed art" the AI can't do anything.

It often takes examples from legitimate human artists to build up its collection and uses those examples to produce images like these

@Potterhead 0-0 : An article here that explains the problem very well : https://www-numerama-com.translate.goog/tech/1221908-les-ia-generent-des-images-mais-pas-de-lart.html?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=fr&_x_tr_pto=wapp

The AI's algorithm searches a pool of artwork posted on the Internet and "combines" them according to the prompts you give it. These artworks are largely used without the original artists' permission. In many cases you can even see the artist's signatures on the finished "artwork". So basically, AI created images are chimeras put together from other people's art, which are then presented as original work and sometimes even sold or turned into NFTs to make money.