Its because they are the "bad guys" and eventually, they have to lose.
Think about it. If you removed the "RAWR, all aliens must die!!" bit, and made the Sith slightly less short-fused, homicidal maniacs, the Empire would roll right over the Republic like it was an old, worn down speed bump. The only point for those facets to be there is so that people will be able to tell the black hats from the white hats, and to make sure that the black hats end up defeating themselves.
Makes sense to me.