Having this done automatically would be like saying that if an image is removed from “The Content” of a post that WP should automatically detach the image from the post or delete the image.
All images that are uploaded while editing a post are “attached” to that post.
In addition to this, if an image is uploaded while editing a different post then it is attached to that post. Selecting or inserting that image on a second post does not change or add that “attachment”. An image can only be attached to a single post and this attachment will never be changed automatically when selecting previously uploaded images.
Showing images or files based on what post they are attached to is prone to issues and you would be better off in the long run by showing a list of files that are actually current in the repeater.
