Who is "they"?
Most AI for math work is being done by AI researchers that are not themselves academic mathematicians (obviously there exceptions). Similarly, most AI for music and AI for visual art is being done by AI researchers that themselves are not professional musicians or artists (again, there are exceptions). This model can work fine if the AI researchers collaborate with mathematicians or artists to understand that the use of AI is actually useful in the workflow of those fields, but often that doesn't happen and there is a savior-like arrogance where AI researchers think they'll just automate those fields. Same thing happens in AI for medicine. So the reason many of those AI researchers want to do this is for the usual incentives - money and publications.
Clearly, there are commercial use cases for AI in all these fields and those may involve removing humans entirely. But in the case of art, and I (and Hardy) would argue academic math, there's a human aspect that can't be removed. Both of those approaches can exist in the world and have value but AI can't replace Van Gogh entirely. It'll automate the process of creating mass produced artwork or become a tool that human artists can use. Both of those require understanding the application domain intimately, so my point stands I think.