Attribution is the whole product
When somebody reshares your post to their story, Instagram renders it as a card: your image or video, your username as a sticker, and a tap target that leads to your post. It is a link with a face on it.
That is a materially different object from a like or a send. It is public, it is attached to another account, and it lasts twenty-four hours in a surface people check constantly. For the duration, your handle is being displayed to an audience that is not yours, by an account that is not you.
The value follows from that. Reposts are worth ordering when the goal is exposure of your name rather than inflation of a number, and they are the wrong tool when you simply want a post to look more popular — because the repost count itself is not what visitors see.