Monitored Folders tells MB to scan the listed folders for new items going forward. That doesn't show all the folders that contain library items already.
The right way to do this is to be in the Library Explorer (Music Tab). At the left, in the drop down, choose the Folder Name entry. This lists all of the folders that contain music in your library. Just each folder with the actual music, not any folder above it in the hierarchy.
To see the full path, display the path in the main panel. You can sort by it, etc.