You're ignoring the fact that the most harm of bullying is not physical harm, but psychological harm.
People are naturally social creatures, and one person insulting you repeatively has a very negative impact on a person. Eventually, you start to wonder if this stuff is true, and that's when the bullying really hits home.
And it's not as easy to avoid as you would think. All it takes is one occurance to possibly harm another person. To avoid this one occurance, you would have to isolate yourself from the very beginning, which is not going to happen because, for the majority, people use the internet to socialize. Frequently, this "repetetive" bullying, is actually multiple barbs thrown by one or more people in one occurance. It's also very common that the person cyber bullying you, used to be a close friend or somebody you know. This makes it all the harder to avoid/ignore them.
Regardless of the medium, bullying is always a serious issue. Regardless of whether or not you think you wouldn't be bothered by the bullying, doesn't mean somebody else wouldn't. It's a very painful experience emotionally. I speak from experience unfortunately. Suspected to have contributed to my anxiety and depression.