Maybe because we've been lectured since kindergarten that boys will be boys, that somehow we must have provoked whatever happened--if it happened at all--latter premise that we're making up fake stories for attention. Then if some boy does something to us we don't want it's because "he really likes you" and we're instructed to be considerate of his feelings instead of our own. Making a fuss about mistreatment gets swift punishment, the mistreatment itself rarely so. Unless some guy notices, decides to play hero, and really overreacts (leaving us to bear the fallout for his overreaction as if we were in control of him).

The people around us don't always respond so badly but it happens often enough that by the time a girl reaches adolescence she already knows it's dicey to seek recourse. Not everyone internalizes the blame but many do; it's human nature to accept what you're told from a young age and it gives the illusion of control to believe that somehow you could prevent it happening if you took all the right precautions.

Even for those who don't internalize blame, there's the practical consideration that formal complaint takes time and effort, probably means enduring grossly inappropriate insinuations and questioning, has little chance of success unless the circumstances were perfect (multiple witnesses, etc.) and very well may result in lasting social backlash against us by getting labeled as troublemakers or liars or drama queens. Path of least resistance is to bite one's lip and get on with life.