I think we've solve the time traveller paradox. The reason we don't see time travellers walking around everywhere is because... it's risky and not worth it. The past already happened, everybody's kind of fine, and changing Jenny's Saturday at the park probably isn't really going to improve much of anything. We still have that wedding to go to in May and there's Casper's birthday we haven't bought a present for. Who has time to fix the past when the future barely has time for the present?!