In multiple terminals and in Halo Legends Origins, the Halo rings are all metal with no topographical features at all. But in everything else, they are essentially a ring planet. My point is, the rings were weapons, designed to kill everything, unlike Onyx and other worlds that were havens for bio-logical specimen.
Maybe it was easier to create a usable atmosphere (breathable by humans/Reclaimers and Forerunners) through biological means, like plants?
Could be a safety measure in case a Reclaimer landed on the wrong part of the ring for some reason. They could hypothetically sustain themselves off of the plant life until they get to the Control Room.
They were also studying the Flood on the Halo rings, so they probably needed natural environments to conduct some of their tests.
because they were used to save species from the firing of the halo arrayPrimordium spoilers
“However, if one observes the holograms of the Installations after stopping the Ark, one will see that the rings have different surface features and may have different atmospheres as well.”
The Fortress Worlds were, and plus that wouldn’t explain why the Halos were ALL metal when they were originally fired.