Because they could kill other people with them? I don't know, what do you think? I guess that's just like email, right?
Children need to be trained in the way they should interact with the world, not let loosed to figure out how to do whatever they want to do however they want. The parents need a way to train the kid in the proper use of this tool. The internet is not for young children. It is an extremely powerful tool that you can hurt yourself and others with.
We also don't let children travel the world alone to unknown places, visiting and interacting with unknown people. Why not? Because the kids will get into trouble and harm themselves and the parents will be oblivious.