Because very few things have more than one head, the rest of human anatomy we have in multiples and they’re in all sorts of positions, heads have a habit of being limited to directly on top of shoulders, and a (usual) maximum of one. So there’s just a lack of training data and any attempts to get multiple heads ends up with multiple creatures. In saying that there are workarounds, and it’s not that hard to mock up something basic in photoshop and then AI paint over it.