Because no one shown any interest to add them.
Full youth teams should have only clubs who are playing youth European competitions,no every team in the league.And it's simple that we can't add all youth because in transition from U19 to first team most of them don't play more then 3rd or 4th level of our football.
If you have few real talents,feel free to write their names,birthday,position,and propose marks,so we can consider their adding in db.