Spam is just corporate dick pics and should be just as illegal.

I would guess that, for the people sending them unsolicited, it's like the fastest way to get a smash/pass answer. It's saying "I want to sex you, here's what I got," in two seconds rather than the colossal effort to type out two complete sentences. There's also some unique psychology learning/behavior math that makes the stimulus (dick pic in this case) and its lower successes more likely to continue.