system
Given the following post, respond as if you're commenting on Reddit:
human
[HALO] Why did the Forerunners reduce the species of the galaxy technologically down to level 0?

Like, they created a manner to replicate Humans, but in Halo Legends they even seem to be a lower form of Humanity, like Neanderthals.

Like, just in case the Flood came back or something, why couldn't they have left some kind of chalk-board with basic mathematics laying around at least? (I'm being coy, obviously).

And then, these structures that re-created the species, how did they hide themselves (On Earth, at least)? On Earth, they seemed not to even be known to exist until the Covenant arrived.
gpt
It goes way back. Before the forerunners were the precursors, who were even more advanced and seeded life in the galaxy. For a while they left the Forerunners in charge, but then thought humans were cooler anyway. The Forerunners didn’t like that idea though and surprise attacked the Precursors, wiping them out completely except for some who stored themselves as some kind of dust (yes they turned themselves to dust to not die) that corrupted them and turned them into the flood. Nobody knew about the flood thing though, or this dust, and after a while everyone basically forgot about the precursors altogether. 

But then ancient humans, who were technologically pretty close to Forerunners, found this dust and thought “hey, let’s feed it to our dogs” and thus the flood was born. In trying to contain it they attacked a Forerunner world that had been infected. Not believing humans about the flood, the Forerunners declared war. Fighting both the flood and forerunners was too much and, even though the flood had retreated before the war was over, the Forerunners won. They actively wiped out most of humanity’s empire, devolved them back to Neanderthals (and some other species), and out them on what they assumed was their home planet (Earth, but no one knows if that’s really their birth planet). The Forerunners also had learned the truth about the flood, and thought humanity had discovered a cure for it. So they wanted humans around just in case, and brought a bunch of them to Zeta Halo (the one in Halo Infinite) to do all kinds of experiments trying to find it. It wasn’t pretty, then the flood they had there for their experiments took it over and did even more experiments (or just tortured them for kicks). 

Eventually when the flood came back in full force the Forerunners had to scramble a bit. They wanted to leave more behind for the humans since humans were supposed to be in charge in the first place anyway, but the flood had found earth by then. The Librarian was there until the very last minute trying to save more humans and making sure the portal to the ark was safe, but the planet was lost. They didn’t really have a lot of options in terms of stuff to leave behind while the planet was overrun. Then the rings fired, and the handful of Forerunners that did survive left the galaxy in shame. 

And then sort of ironically, the ship Truth uses in Halo 2/3 had a forerunner AI on it that was heading to Earth for that reason. It wanted to help them out because it had been a naughty boy during the war and wanted to redeem itself. Instead it wound up crashing on the Prophets homeworld. It shut down for a long time, and the prophets (like their whole species) basically built the covenant with that ship at its center. Eventually they woke up the AI, and it told Truth, Mercy, and Regret that humans were the real successors to the Forerunners. So rather than let this get out and potentially destroy the covenant (and cost them their power) the Prophets said “Hey, let’s just commit genocide and then nobody has to know.” And thus began the human covenant war.