Ah, slight correction, only certain places are on top of the leviathans, namely the five major hubs we have listed. However, that's not an exhausted list so there are more out there 👀

But that doesn't mean all islands use them. There are normal islands that were created in various different ways, including tectonic shifts and volcanoes. Avaleci is indeed weird, but it's more close to being "naturally" made. So there are definitely underwater volcanoes and natural activity, though how often is hard to say given the world is wide and vast. So things could be happening and no one would be aware of it. It is also very deep and not much underwater exploration has occurred due to lack of proper equipment (no submarines) and inherent dangers (underwater creatures and monsters that lurk down there).