Re: "I also don't see any involvement of the internet"
The flip side is that those same phones are often the thing that tips off the carers that the abuse is taking place, and then is used as evidence to catch & prosecute the perpetrators. Think how many cases have been reported where the giveaway was a text message seen by someone else. In the past the kids would meet up with adults in cafes and arcades and it would never be discovered at all. The real cause has nothing to do with technology, it is because a child or teenager who feels unwanted by parents or carers will seek out adults who *do* give them attention and want their company. If a parent does not look after their child properly, it won't be long before the child finds an adult who is more than willing to "look after" them, and when the situation is such that (a) child is a willing participant at the time, (b) both parties have a vested interest in keeping it secret and (c) the parent/carer is not too interested in what the child is doing, then electronic evidence is by far the most likely way it will ever be discovered.