When it comes to facial aesthetics, the most beautiful faces are also the ones with the most average facial features. This means none of the features are extreme/outliers (too big, too small, too long, too wide etc), but are well proportioned and balanced. The most average faces also have the most symmetry, which is another big determinant of attractiveness. So now when the images of the faces of an ethnicity are averaged together you'll get a composite image that is more attractive overall than most of the individual images that were used to create it in the first place.