Part of it is that children are highly suggestible. They're also still mentally developing and learning. Therefore, they are at risk of not developing proper social skills and are more likely to get addicted (possibly at the expense of physical activities). But your point is still valid and both of those things are, for sure, problems for adults too. One would hope that adults are, at least, less suggestible (hope, but in reality, it seems lots of adults are pretty gullible and suggestible too, at least in some things).