I get what you’re saying, but that sounds incredibly complicated and almost impossible to do for the thousands of manga and anime they have. Unlike episodes, seasons, chapters and volumes, arcs mostly aren’t clearly defined beforehand and sometimes can span over multiple seasons or volumes with a bunch of sub arcs inbetween. They’d have to actively keep up with all anime and manga in existence for them to know the arcs. They don’t need to do that for seasons, episodes, chapters and volumes. If you want to review arcs, the best option would probably to create a forum entry on the series.