The Precursors had command over neural physics, their structures were literally indestructible to anything besides the Halos. Neural Physics allows them to transport instantly, sense all life in the galaxy, take control over non-sentient beings, and use said indestructible star roads to turn solar systems into massive blenders, and every Precursor and Forerunner can access the Domain, a neural space that contains the memories and experience of every single Forerunner or Precursor to ever live. The Force is something to contend with, yes, but how many force users are actually strong enough to influence a battle in space, especially when the Forerunners can outproduce, outgun, and outthink the SW forces? No matter how impressive the SW ground forces are (hint: not that impressive compared to that of the Forerunners), victories on the planets don't matter when you lose the battle for the system. That was the problem with the Spartans: they couldn't lost on the ground, but the UNSC couldn't win in space, so all those victories were pointless.