We don't have a definitive answer so far but as you mention it, it would cause a lot of changes in the timeline.
Rip takes a lot of precautions around his mission:
- He travel back in time to meet the professor (Hawkgirl's son) just one day before he is supposed to die, so that meeting people from the future will have little to no effect on the timeline.
- One of the big surprises (in comparison to the trailers) of the pilot is that his team is composed of nobodies, with no influence to the original timeline. Despite these precautions, Rip Hunter is consider as rogue by the other Time Masters, who send a bounty hunter to kill him. They justify their "no intervention policy" by the risk that killing Savage could lead to a bigger threat.
The idea of avoiding to change the timeline as little as possible is legitimated not only by the usual butterfly effect, but also because "time wants to happen", as Rip Hunter says, and that "they may have to fight time itself sometimes" (approximative quotes).
Finally, in the show Flash that takes place in the same universe, Earth-2 Flash Jay Garrick takes very seriously the fact that its Earth-1 counterpart Barry Allen traveled back in time, and Barry's significant travels were only one day back in time (he finally didn't change anything when he traveled back to the time of his mother's death, some years before).
To sum up, travelling back to several centuries/millenaries BC seems to be a very risky idea.