The short answer is, because it complies with the MFCF xhier(7) (for no logical reason).
The longer answer is, I like xhier more than the FHS because my distribution is Slackware so I have no idea where the files comes from. If you think this is nonsense, perhaps you could grab the xhier paper and read it. (This reason is indeed very lame because I have no xhier automated support.)