Heres that age -old theory put into MY words :p I had lots more to it, but it was spread about and I couldn't find it all.
My thoughts on why they won't glass planet Earth:
We all know the civenant are obssessed with the Forerunner. They love the forerunner as if the forerunner are Gods or godly beings, and the Civenant are striving to become as powerful and technologically advanced as the covenant once were
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The Covenant believes that the Forerunners, a long-since vanished and utterly mysterious civilization, are god-like, but not literally their gods. Rather, the Forerunners represent a level of being that must be attained, or at least attempted. This belief system is underpinned by the existence of numerous abandoned Forerunner artifacts, objects and installations, of which the artificial world Halo, is just one example. The evident technological perfection of Forerunner objects, and the awesome, and unknowable purpose for which these artifacts exist, has powered the Covenant theology for millennia.
There's nothing like evidence for the existence of god, than finding his abandoned toys scattered throughout your backyard. However, the Forerunner technology is as obtuse as it is arcane. The Covenant can only guess at its true purpose and origin, and much of their belief system is based on speculation and dogma. That said, they have been able to incorporate some Forerunner advancements into their weapon and transport technology.
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We know the prophets are the guys in charge - the brains of the outfit, so to speak. These are the guys who do a lot of the research and direct their minions to destroy races and homeworld. The prophets decide whether a planet is to be glassed (i.e. there being no forerunner artifacts there) or whether to be taken by the ground (giving the impression that there is forerunner technology there).
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The pact allows the prophets to advance their search through conquest and occasionally assimilation of species whose homeworlds or solar systems contain Forerunner architecture or technology, often unguessed at by the resident species.
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Remember that the various covenant races are not at ease with each other. There is a lot of underlying tension and each is not sure of the others motives, so each will keep secrets. The prophets tell the other races that the forerunner are gods and that they must reach their level.
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Elites coexist with Prophets uneasily. They need access to the Forerunner technology parsed and reverse engineered by the Prophets, and certainly demonstrably share some religious beliefs. But the Elites prefer to demonstrate their faith on the battlefield not the temple. They have little patience for those who believe that liturgical pomp, rather than conquest, is what the Gods require.
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Another piece of evidence that shows tensions and secrets and perhaps deception and betrayal is:
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They communicate only with Prophets - a fact that may contribute to the balance of power between Elites and Prophets - and in fact will usually ignore anything that is neither a Prophet nor a Forerunner artifact.
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Now, lets think for a minute. What if the forerunners are ancient humans? Please, remember that the covenant are trying to obliterate every single human and every single human colonised planet. Why would they do this? Not every human planet has forerunner artifacts. Reach didn't have any. So why to the covenant strive to obliterate us? Well, I'll tell you why. The Prophets and perhaps the engineers know that the humans are really forerunners, so this destroys all their beliefs about gods and getting so powerful as to be in the next dimension. If humans are forerunner, ir shatters their entire system. So what do the prophets do? They keep this a secret from the other races. Because, if the elites or hunters knew that the forerunners are humans, then they would turn on the prophets and everything the prophets have strived to acheive would be lost. The elites and hunters etc blidnly follow the prophets orders, glad for conquest and blood spill. If they knew, the prophets may lose their leadership to the humans.
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They have little patience for those who believe that liturgical pomp, rather than conquest, is what the Gods require.
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Now, how can they be forerunners, you all rightfully ask? Well, their was this virus, dubbed the flood, which started eating and killing every forerunner/human (from now on called human). The plague destroyed planets full of humans, killed everyone. But lets imagine the humans, on one of their planets or ring worlds or space ships sent off one large life ship. On this ship was a whole bunch of humans who would escape the flood, and who would flourish and re-build the human empire. This ship landed on Earth. Over the years the humans lost their technology and went back to being primitive, and now they have become advanced again, not as advanced as before, but they're getting there, and none of them know about their true past.
Why won't the covenant glass Earth? because there is tons of 'forerunner' or ancient human technology deep under the ground there, waiting to be discovered. Waiting for the humans to discover their true past.
Now, more evidence as shown in Halo: Combat Evolved:
343 Guilty Spark speaks the wonderous language of English, though I highly doubt the ancient humans would have.
343 Guilty Spark gives a lot of hints away, at one point he even mentions that Master Chief looks like a forerunner!
343 Guilty Spark asks why MC doesn't have a Mark XII suit, commenting on his dated Mark II suit
343 also calls him a reclaimer, mentioning that he's "done it before" etc.
Environment on the Ring World shown in Halo: CE is incredibly similar to Earth's environments.
Gravity of the said sing world is implied to be the same as Earths.
Sky is also the same, as well as the "seasons".
You know it makes sense!
All quotes and sources are directly quoted from an official bungie source, of course.
There was more to this, but I lost it in various places :( So I decided to immortalize what I had, here.