1. Time travel's not possible.
2. It's not possible in a corporeal manner.
Or if it is possible:
3. We don't live long enough to discover it.
4. Illegal to "out" oneself as a traveler.
5. Paradoxes make it impossible to out oneself.
6. It's outlawed in the future.
7. It's possible, but so difficult/expensive as to not be practical.
8. It has already happened and Bush is actually a Neandertal escaped from our distant past.
I'm voting for number 8.