What work do you think mathematicians do normally?
Like they sit whole day and have ideas? And where are the ideas?
The way I see it, _some_ mathematicians enjoy solving puzzles, and now AI is better at solving puzzles.
This does not affect people building new theories.
Also, it's quite prestigious to write a _book_ on some topic. And guess what writing a book entails? Refining and expanding. What you call grunt work.