The reason why this is true is because of two things: 1. Lazy-ass people who want a space shooter- ready...GO PLAY DOOM, YOU LOWLIFES! 2. people who cannot afford them, like me. I cannot buy the Forerunner books because I need to earn more money, but I HAVE READ all of the other books, including the HELLJUMPER book.
To dumb it down...
The flood- originally purposed to be an ETERNAL HARD DRIVE; keeps all info safe and unable to be harmed. Also has a failsafe built in- wants to keep getting information, but cannot do so directly, so it has to communicate/absorb a host. Flood is eventually like a COKE ADDICT on withdrawal, and has nothing to lose. Attacks anything thinking to absorb data/ intelligence. The Gravemind is like the main server, which needs to be programmed before it can be used.
Precursors- the supposed "gods" of the Halo series- created the flood, and all kinds of life (forerunner, human, etc;) all are SUPPOSEDLY dead now.
Humans of old- strong, desperate, and almost forerunner intelligence, humans were fighting two wars at once- against the flood, and against the Forerunners. They basically genetically suicide-bombed the Flood through genetic mutation of their own population; the weakened humans then fight for new lands from the Forerunners because they lost much land to the flood. Forerunners then make them lose all knowledge, so they cannot fight back against the flood. Only one known human of this new era survived- the human Chakas, A.K.A. 343 Guilty Spark.
Forerunners- the ignorant, yet supremely advanced race of the era; attacked humans without prejudice to defend their lands (and conquered easily, I might add), yet could not figure out how to stop the Flood, so they created the suicide food-denial Bomb called a HALO; they were used because forerunners couldn't figure out how humanity stopped the flood, and the Offensive, Defensive, and Mendicant Bias A. I.'s failed. The only group known to use a device like the Composer to create A.I.'s