Because (if they're under 18) they want their 'boyfriends' to be arrested on possession of child porn and be classified a sex offender.
Ok probably not, but that's what can (and does/has) happen. My brother is a high school teacher and 'caught' a guy with 'indecent' pictures on his phone of his 'girlfriend'. There was a big sit down with the both kids and their parents, and also the police. They explained the law to him (both pictures and age of consent, as she was 15), but also showed him (with a different phone) how even if he deletes the file, savvy tech people can restore it. (This was important, because it was to show how if he deleted it and sold /gave away the phone, others could get the pictures back).
He was lucky in that the police used a bit of discretion and thought a bit laterally, but as I wrote, some people have been arrested for it.
So, if you're a girl and you're into this, realise the risk you're putting your Boyfriend in.