To give some background on why the Precursors disappeared... The Forerunners were one of their many creations; they created life for fun. Once they decided it was time to try out another one of their creations, the humans, the forerunners opposed their own creators. Awed by the forerunner's stubbornness and destructive tendencies the Precursors decided to not hold them back, so they could watch chaos enfold. Basically after that, the last one of the Precursors decided, to give a damn, became the Flood (because why the -blam!- not become an infectious hivemind space parasite for fun?) and curbstomped all of the Forerunners by himself and humiliated them as well made them break all their philosophies during that. And I should mention that the Precursors literally cheat death; they are simply a higher instance that can come back to life as anything. In their past they deliberately killed themselves just to try out another form.