Those prosecutions are rare because they are hard to win. If there were a reliable way to prove that a picture or video involves someone younger than 18, those prosecutions would be far more common. It is not as though the prosecutor receives a case of someone has pictures that appear to be of minors, and they say, "Gee, I think we can let this slide. Those girls are clearly sexually developed!"

At the end of the day, a prosecutor's career depends on their ability to win cases. Prosecutors choose cases they believe they can win, regardless of whether or not those cases actually make sense.

Thanks for confirming what I wrote earlier. If I drive 3 miles faster than the speed limit, I usually won’t have the highway patrol chasing me, but that doesn’t mean I’m not breaking the law.

Just because you don’t consider it child porn if it’s graphic depictions of persons over 10 years of age, that doesn’t mean it’s permitted by law.