This is the same argument that many people used with "Heroes" - why didn't Hiro just go back and prevent everything bad from happening. You could ask that - why doesn't Ben just go back and stop Jack from calling the freighter, or going back and stopping Annie from dying(?), or stop his dad from going to the island, etc, etc, etc.... It wouldn't make for a very good show though. So far - we've only seen Ben travel to the future, not to the past. We've only had flashbacks to show us the past. Maybe you can't go to the past, only ahead and then back to the island at the time you left.

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