Basically because of what they can do. They have abilities but most of the wonderful animations are on water.... base game worlds up to Pre-Island Living worlds (except evergreen harbor) has no Swimma...
I kind of want to make a pool for a mermaid just to see if they can summon a dolphin in the pool. It wouldn't surprise me if they could, it's sims. In the Sims world: "What's logic?"