It's because the average person see's every rapist ever, as some sort of inhuman monster
Serial Rapist =/= Rapist, you'd be surprised how many incidents are singular, and the majority of victims are raped by people they know for reasons that aren't always exclusively about violence or control.
But of course the above paragraph is heresy
It is better to believe that every single act of rape ever committed was done because the offender seeks to destroy the sanctity of virtue, and is nothing more then an inhuman monster. Rather then somebody who might just have an emotional problem and committed a crime of passion (which is still not an excuse, but who hasn't committed a crime of passion).
Of course, everyone who's ever struck a human being, must also be executed, or horribly mutilated, because that is a crime of violence and control, and never a crime of passion or a horrible mistake.