Why the face stays human
A full robot head is easy to generate and hard to make convincing. It also removes the person from the photo, which defeats the point of a costume portrait. This preset explicitly says not to replace the face with a robot head. The costume frames the face instead of hiding it.
The second constraint is believability. The armor should look wearable: panels that follow the shoulders, seams that suggest assembly, and LEDs that sit on the surface rather than floating. That is why the prompt asks for a costume worn by a real person, not a new mechanical character.
