[So Cal Rocket]

If time travel into the past is possible... then it's reasonable to think that at some time before the next millennium, someone will have found a way to do it and perfected it... and if they're able to do it, people would have come back in time and would be noticed. This hasn't happened...

So, this leads to three possibilities:

1. Time travel is impossible.
2. Time travel is possible, but the people coming back in time have been very, very covert about it.
3. Something cataclysmic happens that stops scientific progress before time travel is invented.

[Daffynition]

Lookie here: Professor Predicts Human Time Travel This Century