Despite the insistence of a couple generations of shadetree mechanics and even its own manufacturers, the proper nomenclature for the material in question is "duck" tape, not "duct" tape. It was never intended for, and performs very poorly at the task of sealing ducts.
"Duck" is the name of a type of densely woven cotton cloth, treated with wax. The waterproof nature of this cloth is where the name comes from, as duck feathers famously repel water.
The original duck tape was made by applying a pressure sensitive adhesive to strips of duck cloth. Modern variants still use a fabric matrix for strength, but have added a layer of plastic to provide water resistance.