I believe in the very beginning they started by using the free public-domain artwork. And it caught on, so they wanted to continue, but frankly anybody can subs

I believe in the very beginning they started by using the free public-domain artwork. And it caught on, so they wanted to continue, but frankly anybody can substantially copy their cover by just using the same artwork. So it makes sense to create new artwork each time that has the same look but can be protected by copyright.