Monsters are fun. Whether it's a demon, genetic freak, lab accident, or just some kind of alien that happens to look really monstrous, the creators want to show this monster in all its glory. This means they will wear as little as possible to really let the audience see how ugly they are. Of course, they usually can't get away with a naked monster running around, but they want to avoid Nonhumans Lack Attributes for whatever reason. What do they do instead? Why, they'll slap on a Loin Cloth, pair of underwear, or maybe just some pants. The monsters in these situations are almost always sentient but decide to run around wearing as little as possible, rarely with anyone saying anything.