Rape involves force. Statuatory rape does not involved force. WTF?
Well, that is the definition of the term "statutory". Even if the act does not explicitly qualify as a crime, we still hold it as a crime because of whatever factors might be involved. In the case of statutory rape, the child's age means that we ignore virtually every fact at issue.
For example, a 15 yr old girl and her parents could all tell a 24 yr old that she is 19, she shows a (fake) ID, and she instigates all of the "action". There is no "mens rea" (guilty mind), and no forcible act... but the 21 yr old can still be easily convicted of rape. (And yes, this has happened in my state, Indiana.)
Again, for all "statutory" laws, we basically ignore the facts, and call it a crime, just because of XYZ.