In many ways, it isn't. Putting the stuffing straight into the turkey means it needs to cook longer, because less of the hot air can actually get into the cavity to heat up the insides. Longer cooking times, then, mean a drier meat.
So how can we get delicious, moist stuffing without sacrificing the turkey, or moist turkey without sacrificing the stuffing? Simple: Cook them separate, then combine later.
Cook stuffing outside the turkey in a regular baking dish, then set aside. For the turkey, take a large, empty can (both sides cut out) and stuff it inside the savity, then cook as you normally would. Once the turkey is done, push the stuffing inside the can, then pull the can out and leave the stuffing behind. Allow the turkey to rest for a few minutes (and if you want to be super coy, brown the exposed bits of stuffing with a blow torch). The juices inside the turkey will drip into the stuffing and you'll end up with perfectly moist turkey and stuffing.