ITT: Why have we not met time travelers in the open?

1. Not possible to physically perform
2. Using time travel to go into the past, alters/destroys possible future outcomes, which could include jeopardizing the time travel machine.
3. Moral and ethical limitations. (I highly doubt that matters. It would be used, if invented, certainly.)
4. Requires some sort of interaction in the present, for future people/beings to achieve getting here.
5. Perpetual nemesis. (The first person to use time travel, can continually thwart/kill anyone else who is going to develop it independently. This would maintain a monopoly on time travel. ie: Primer)
6. Could be here already, and are just staying low profile. (I feel this one is garbage, but why not.)