Because mixed media does not exist.

Nothing forbid anyone to train an AI with its own drawings in its own style.

Once again, AI is a tool. Like many others used in digital art. It's just a statistically driven generative algorithm. People can use a tool as they please to make art, same as they can use any other tool, and you have not the authority to gatekeep an artist of doing art just because you think their tool, their style, the object or anything about the artist does not fit with your morals.

And they also can, and will, mix it with other tools to produce the piece of art they want to create.

Also all this discussion about "the style(tm)" could be just disproven given the fact that if you weight your variables and use a specific dataset you can generate consistent images in a determined style. And some AI artists does have a representative style due to this... So...