very old topic here but showing that creatures behavior and such are planned to be different and current state of things is temporary (so all creatures spawning everywhere); there will be creatures in future that inhabit caves and underground (spitters and cuttles spawning there now is a placeholder test) and water creatures are planned as well;
it’s the same with weather - currently all types of weather can happen in all biomes but more biome bound weather is in making (snow in ice areas only etc.)
anyways, here is link to older discussions and ideas around sandbox creatures; you might find it interesting