Why, I don’t know. It’s just the way it is. If it isn’t depending on embedded metadata, Plex uses the folder structure and filenames to identify the tracks/albums/artists. That much makes sense. But even when Plex is configured to prefer embedded tags, it still depends on the structure. You can find a number of threads in these forums about this.
https://support.plex.tv/articles/200265296-adding-music-media-from-folders/
