People that young cannot give consent because they are not mentally mature enough to make that decision. It's the same reason people that young cannot sign contracts.
I'm not gonna argue with this. The argument is the definition of "rape". There is a distinct difference between an aggressive rapist who forces someone to have sex, and an overage person having sexual relations with a willing, albeit underage, person. To call both "rape" makes no distiction between one and the other, where there should be a distinction.