I imagine it's not super easy finding the type of models that they use. A) they need to be cute, and 2) they need to be willing to show quite a bit and have parents/guardians that will allow it. There are alot of variables there.
You're missing the crucially important point iii) they need to be reliable over an extended period of time for new shoots. This is where a lot of sites fail, and why the infamous Jimmy just gave up - teen aged girls are flakes. They miss shoots, blow off scheduled chats, ignore emails, and are generally a pain in the ass.* The TS guy has had a lot of problems with this, and the wreckage of all his other single girl sites reflect it. Maybe he just isn't good at communicating with girls that age**, maybe he isn't the most organized and the girls make it worse, who knows, but his relationship with the girls and/or their parents fails and another domain name goes dark. Starting with yet another new girl is a big money risk unless the photog can get the girl for a solid week of shoots and have enough content to run even if the girl disappears the next day. (See: Dawson Miller. Every single photo was shot in one week, after which the model rediscovered God and bailed out. And that site ran for
years before the webmaster started repeating.)
I'm sure the guy running TeenStarlet would absolutely love to add a new young, hot, long-term moneymaker to his websites. I'd like to win $30K with a $1 scratchoff ticket. I'm not sure who's odds are worse.
* No doubt teen boys are worse, but who fucking cares, I don't want to see them model.
** Unlike the legendary Andrew Slater, a former high school teacher, who could talk the shyest teen out of her panties on video and still get her to show back up for another shoot on a later date. A total creep, but you have to respect the skills.