Because it's their work being used algorithmically to support someone else's.

Regardless of how you feel about AI, the training models have to exclude copyrighted works to not have this happen, because otherwise it is absolutely true that that AI keeps record of everything fed into it, and if you dont have the rights to what was fed into it, then there's a copyright issue. Because even if it's being reworked and influenced by other works, it is still using other people's stuff to do it. It is, in many ways, an overgrown randomization & automation tool.

The problem is that people dont see AI's as a tool that companies are using, they see it almost like a person learning. It's not like a person learning, and cant be treated the same as say, a consumer reading the book referenced (in this example) for enjoyment.