Heh, you undid Buddy at the same time I did. Basically, as you can see, it's because it's not really relevant to the info box and we cover far too many movies (especially all those PG-13 to R movies where the relevance is just some Creature Shop effects or props) to do it every time (and then it gets into near semantics, like for all the G ratings, where we might as well say "G for everyone" or even cases where no rating was applied or added later for video, etc). I wondered why you did that, and then thought to check The Muppets (2011), so now I see why. I see there's an explanation there, but that's an exception because there was uncertainty about the rating and it's still new out (so people would be more apt to think "Why is the new Muppet movie not G?" than they would to wonder about The Witches or anything). Personally though, I think it would be better to discuss the rating in the text, but that's something we can tackle on the article's talk page later (the page is still more of a work in progress, after all).
Plus of course those rating explanations have become more detailed over time (for the original release of Labyrinth, the best I can find is "thematic content," as opposed to "scary scenes, some sexual implications or subtext, infant in peril, and David Bowie in suggestive tights"). In general, the reasonings of the MPAA are outside our ken and focus anyway. Not that you needed the explanation, but here you go anyway!