Halo Silentium, the Primordial Gravemind sends a ghost of Forthencho, the Ancient Human Lord of Admirals, to tell the Librarian one final secret. The Gravemind, through the Lord of Admirals, informs the Librarian that the Precursors had "come from a time before there were stars" and had amassed a repository of over 100billion years of knowledge hidden somewhere in the galaxy. This repository was the Domain, a mysterious timeless Ethereal connection to the universe the Forerunners had been using to store and access memories and meditate. As the Domain's physical anchor was a Precursor Neural Physics structure, the Halo pulse would destroy the Domain, and with it, the Forerunners' entire 10million year history and culture; thus, the Forerunners would be committing spiritual suicide. The universe is 13.7billion years old. The Precursors (who are often referred too as "Timeless") have existed for over 100billion years and stored that history in the Domain, itself a living, thinking entity of unknown power and intent. As such, the Precursors are older than time, at least certainly older than our universe by several times. Hell the forerunners are older than Abeloth, with ages of tens of thousand of years being quite common to Forerunners, who were functionally immortal thanks to technology. The Precursors however are not mortal; even killing them only mildly inconveniences them, as they will just self resurrect.

Is that why they got whooped by the forunners and didn't come back to life scrub stars wars all the way and I'm a halo fan it's just a simple fact that star wars wins in the long run plus the flood wouldn't help the halo universe it would be a threat to both universes