Maybe it's because it's what they have knowledge of?
[He pauses, his tone taking on a more contemplative quality, like this is actually something deeply interesting to him.] Throughout history, people have tended to call things they don't understand with names that already mean something to them. If I didn't know better, I would think you are a seraph, using seraphic artes. And if you didn't know Mikleo and Edna, you could have thought they are bending water and earth.
Just like that, those who know "magic" as everything that can't be explained as something natural... they would think both bending and seraphic artes are magic, because neither of those things exists in their world!