Because hentai is made for men and a lot of men love jailbait and jail-bait looking females. This is why Hugh Hephner dates women young enough to almost still be in high school. They are models and models are not that bright, so maybe they still are in high school.
If you look at a lot of other porn you will notice this too with a lot of school girl themed sites, Sites with the word teen in their name even though most of the models are in their 20s and a lot of porn videos have the adult film star wear a catholic school girl plad skit.
Even our own president likes to look at jailbait ass. Its almost like you can read the word "Daaamn!!" starting to come out of his mouth. While France's president Nicolas Sarkozy is trying to be discrete while checking her out, but he still looks like he got that shit eating grin on his face
http://tizona.wordpress.com/2009/07/11/jailbait-obama-ogles-brazilian-beauty-booty/
http://www.urbandictionary.com/define.php?term=shit+eating+grin
(1) n. A shit eating grin is a very wide and, to the outside observer, stupid looking grin, usually showing smugness, self-satisfaction, or inner humor.
or
A)The grin on someone's face when they are trying not to smile but cant stop so they look like a total shithead.