But the Star Wars universe is so fractured, by the time the flood would be seen as a threat it would have space traveling technology. That is why the forerunners eventually lost. The humans were attacking planets trying to stop the spread of the flood and the forerunners thought they were being aggressive and greedy. The flood needs only one quiet infection of a planet with space faring technology. After that they can travel any where. If they start in the fringes of the galaxy and take a few planets not part of the republic, Game Over.