Because semi-intelligent floating bits of elemental power don't strike me as the kind of thing that needs to breathe, and because drowning traps for them are absurdly cheap and cheesy. It's unfair to the wisps and just turns them into a resource farm with even less effort than a mob spawner when they're supposed to be a not so desirable side-effect to generating too much flux. They're supposed to be a hazard (at least the hostile ones), not free alchemy ingredients.