I think the proper question is “Why wouldn’t they?”
The answer: Confidentiality laws. There are many reasons why they would be unable to report sexual abuse.
Another thing to take into consideration is does the girl want to report? Sexual assault is traumatizing, and in reporting it, many rape survivors face all kinds of scrutiny, and many are re-victimized on the stand, are called liars, are treated with unkindness. There is so much to take into consideration when you want to prosecute. Will this person retaliate? Can I handle talking about my abuse? Will the judge be fair? Will the judge be unbiased? Will the defense call me all sorts of horrible names? Will they judge my appearance, lifestyle, habits, location, how much I’ve drank, ect? Some judges are notorious for looking at young rape victims and chastising them for their assaults.
I’ve never reported my rape. Neither do 60% of rape victims. Forcing a rape victim to prosecute is wrong for a lot of reasons, and can be almost as traumatizing as the actual assault.