They are paid for the same reasons the NEA pays artists: out of a sense of obligation to demonstrate elite culture. The track record of practicality of pure math after WWII is essentially 0.
Similarly I wouldn't expect a good argument could be made that AI tools should be prevented from creating art because we want to continue funding artists.
If the goal of said funding is just to let them spend their time doing it then it doesn't matter that AI is doing it as well.