My guess is that you have never received unsolicited boob pictures because women fear to look like "sluts". They might think you could show their pictures around to your friends. That's a big reason why many women only send nudes to their partner, or are advised to send them to their partner only. They need to trust the person on the receiving end, feel comfortable and know that man isn't going to make derogatory comments about their bodies . They don't do it randomly. (But these are just my assumptions anyway.)