The Grave mind isn't building the Halo. It appears to have been activated by a Sentinel AI, because the previous one (From Halo CE) was destroyed. You'd think, after playing through the previous two levels (on the Ark), that you'd come across legions of Sentinels. The Ark should be defended by millions of the flying hoverfreaks... and it isn't. It's just this empty barren place with nothing on it bar the Covenant and eventually the Flood. Hmm. I wonder what Bungie was thinking?