Because, thanks to the Feminist movement, and the politicians who pander to their selfish interests, speaking of women in any other context where they aren't portrayed as victims is seen as "misogyny", and such debates are silenced by accusing them of encouraging anti-female "cultures" [like "rape culture", for instance.]
People base their decisions on raw numbers [which show that more girls than boys as victims], while at the same time conveniently ignoring that most boys who are victims of sexual abuse at the hands of older girls do NOT come out and report such instances [out of shame, or by thinking that they "got lucky"].
This situation will change only when instances where the perp. is female and the victim is male are reported often [as and when they occur], and when we aren't scared of highlighting such instances in public.
The stereotype that women can do no harm must be broken down before a radical change as the one you expect is made possible.
Good question.
ETA: Again, there are certain common misconceptions about how female and male perps. are treated.
While a male perp. is usually sent to jail and expected to register on sex-offender lists [which is definitely fine in my book], female offenders receive more lenient punishments for the same, or comparable crimes. Look at what happened to Debra LaFave - she was ordered to seek therapy. A male offender in her situation would be sent to prison for a very long time.