Ancient humanity actually rivaled the power of the Forerunners. During the human-Forerunner war, human tech advanced to extremely high levels. Enough to effectively contend with the Forerunner Warrior-Servants for 1,000 years. The Forerunners were extremely arrogant, so when they encountered the Flood, they treated it like a disease. By the time they realized the true nature of the parasite, it was too late. The Flood had already gained a too significant portion of the galaxy. This is further compunded by the fact that in the final 10 years of the war, they gained access the invincible Precursor constructs like Star Roads.