Back to the actual question, though, and it's really just that the parents aren't reporting it in many cases. If they think she's ready, and he's not a 40-year old creeper, they don't do anything. Probably figuring that doing so will just push her towards him even more, rather than breaking them up.
And then you'd probably have a hard time selling a jury on rape when they see two kids that are just a few years apart, and seem like they love each other, it's consentual, etc. Legally, it's technically wrong, but may be difficult to succeed in court.