When you share links with people, many (most? at this point) include previews. Those previews are performed on the sender side, e.g. the person sharing the link

When you share links with people, many (most? at this point) include previews. Those previews are performed on the sender side, e.g. the person sharing the link. The "Sharing" view is part of macOS and iOS, not part of the browser - any other browser or app can also use the sharing pane and also be a recipient of the information being shared. If the recipient service/sharing extension needs or supports preview images, then the sharing view produces them, which requires network loads. In this specific case there isn't even a question of "can you use, or is it appropriate to use, safari's local render?" because no such render exists.