I think it's because it's an exciting and visually appealing way to showcase a woman's commitment to building muscle. It shows that women can be fierce and focused on their fitness goals. Maybe it's also to break stereotypes and encourage more women to get into strength training.
Well, sometimes comic characters can be featured on jewelry, and stamps can have comic-themed designs. But generally, they are separate fields with their own purposes and audiences.
Well, a black hair manga boy could have sharp features, intense eyes, and a brooding expression. Maybe he's a silent but powerful type or a complex character with hidden depths.
Well, in the 'Koushaku Reijou' case, the anime may condense or expand some storylines for better pacing. Also, the character designs might have minor variations in appearance and expressions.