Why does Instagram only tag someone when I send them a collaborator invite?

An Instagram collaborator invite is different from a regular tag, and it doesn't become a shared post until the other account accepts it. Instagram puts both options under Tag people, which is why it feels like the invite is just tagging them. A normal tag links their profile to your post; a Collab invite asks them to co-author the post or Reel, and once they accept, it can appear on both profiles. If you only want a normal tag, stay in Tag people and add the account rather than choosing Invite collaborator. If the invite is still pending, give the other person a chance to accept it before assuming it failed.