As awful as it sounds. The young boys are viewed as an investment in the hopes they succeed and can play for Leicester or generate revenue via a sale to another club. This market doesn’t exist for the girls, which could be a factor in why the boys play free and the girls pay. Again sounds awful, but unfortunately this is how it is currently.
Edit: I’m aware I’m a moron and this is probably not correct. Was just the first thought that popped in my head