Why can't you restrict user access by folder?

Plex allows you to browse content by folder, and everybody I know categorizes their movies\shows in different folders. So why simply restrict users to libraries, and then allow certain ratings? This allows some movies to slip through the cracks, not all PG, or PG-13 movies are the same. For example to let my children have access to Shrek the Third, they now have access to TRON: Legacy. Its not a big deal, but I’m just surprised with the lack of granularity.