I’ll add a little about what makes me see it this way. “Art” is so individual, but in my view it is a model of perception. What is beautiful to one person is a mystery to another. Is a canvas that to one person has had paint spilled on it more beautiful than a watercolor of a recognizable landscape or the detailed “correct” work of a da Vinci? None are “real” and all are perceptions of reality.
When I see a tree, I see beauty. I also see fractals, and tesselations, and cones and cylinders and cones on top of shells of cones. When I see Vitruvian Man, I see geometric figures on a Cartesian plane. When I see a roller coaster, I see functions and transformations of functions. When I see flying buttresses, I visualize the buttresses on trees and the engineering and architecture of the trees. I am a quilter and about 10 years ago I was given some fabric and tasked make a particular quilt. I wanted to start in the center with a “landscape” orientation panel but needed it to fit a “portrait” orientation bed. Geometry and arithmetic made it work out pretty well.
A summary of how I think of math as an art might be that any model (stuffed animal, sculpture, lego structure) is a form of art. Some of us see the “art” in the form of a mathematical model or equation (or statistical model, or empirical model, or mechanistic model, or…), but is that any less of an art than the sculpture or the painting or is it just a different form of art?