I would venture to guess that in order to properly study the Flood, like they stated in Halo campaign, the forerunners would have needed proper atmosphere in order to survive. Many of the cold areas of Halo were also there for research.

From what 343 Guilty Spark says in the Halo 1 campaign it seems that the forerunners were either humans or amazingly close to humans (I havent beat Halo 3 campaign no spoilers)

Plus all of the underground areas was meant to hold the Flood in. Halo needed to be huge to generate the necessary power as well.

The whole Halo Story fundamentally doesn't make sense. So you kind of just have to ignore that part and enjoy

Partly to blame is just poor storytelling.

Honestly though, do the majority of Halo players actually care?

Poor storytelling your kidding right?

And what doesnt make sense?

Pretty much everything after you select the difficulty and before the credits roll.

Seriously though, in my experience Halo 2 actually seems to have the most easily understandable story of the 3.  Halo's original storyline just didn't leave a strong impression on me.  I remember navigating those stupid environments that gave almost 0 clues where to go far more than the story.  My experience with Halo 3 is limited but I'm not impressed.  It's not a must-buy for me.

I'm pretty sure its all explained somewhere. Read the books.

It was done to make money for Microsoft.

^ yes because when the creation of halo started out for the mac it was all apart of Microsofts master plan to take your money.

the forrunners lived there and studied them, made it like their home world, till they fucked up and had to activate halos, halo does not kill the flood however, just denies them food

What I don't get about the Halo storyline is generally everything about the flood. The forerunners were supposed to be this relatively ancient race that was super intelligent and powerful, but their final solution for dealing with the flood was mass murder and suicide? Eradicating all host life was the only answer to stop the flood?

It only took Master Chief five seconds to find out that a shotgun does the job just fine.