The difference is that images of violence aren't as strongly associated with things like sharing of personal images that haunt a person later, with really disgusting and effective forms of bullying, or with efforts to get kids to do something harmful IRL. Could they be? Can I concoct such a scenario? Sure, but these other things are in-fact happening a lot (ask any teacher or other person who works with kids ages ~12 and up)
That's why specifically when it comes to Internet messaging services and kids, nudes are a bigger problem than images of violence. I agree with you more broadly, but the context is important here, and is why in this narrow case the two things are meaningfully different, and nudity is the bigger problem.