Most teachers are experts on the particular topic they try to educate their students on. But they aren't necessarily experts at teaching. It's assumed that because they know the topic well, that they can teach it well, but most teachers (at least the ones I've had) can't. They don't know about which teaching techniques work best or which kinds of assignments students actually learn from or what kind of atmosphere a class should be.
In contrast, artists are experts at a particular craft and use that craft to express messages. But they aren't necessarily experts on any other topic and may not have anything intelligent or useful to say with their art. They're ordinary people that have platforms for personal expression like bloggers - and like bloggers, they don't come with a guarantee of being well-read, smart, original, or moral.
Teachers have things to say but can't necessarily communicate those things effectively. Artists can communicate their ideas but don't necessarily have anything interesting to say.