I mostly agree with your sentiment, but saying "math is not art" is the same as saying "writing is not art". Calculation isn't art. But math isn't calculation. Math is a social activity shared between humans. Like writing, much of it is purely utilitarian. But there's always an aesthetic component, and some works explore that without regard to utility. It's a funny kind of art, accessible to few and beautiful to even fewer. But there is an art there.