I was reading this on an article, and I have actually experienced this myself too. But why is this?
Most Helpful Guy
I'd guess is because nude takes virtually 0 effort to receive with 0 impact on their life.
Sex you have to plan and implement. Even if you were to just show up, it still takes time up and there's risk of STD, pregnancy, or simply you knowing where they live and they just don't find your juice is worth the squeeze.1
Most Helpful Girl
If the girl sends the nude (normally happens) and she's dumb enough to put her face in it (massive mistake) then he can blackmail it, and he can keep it forever.
If you're going to take nudes never put your face in them.1