Why are most maniquins skinny?

I know we need mannequins to show off the clothes they are selling but why are they always the same size and proportion?

I saw an animation (I don't remember the animator's name), but the girl in that video wanted to buy a red dress she saw on a mannequin but she didn't look like the maniquin so she went on a diet and eventually gained a severe eating disorder leaving her deadly skinny and passing out on the street after still not being satisfied with how the dress looked on her. To one side she saw a bunch of mannequins wearing different clothes. Two or other side a bunch of real people who were genuinely concerned wearing the same clothes.

Why aren't there any plus size mannequins with real bodies, wide shouldered mannequins, short mannequins that aen't kids, Things like that?