As others have said, they're colony ships. Clearly, only an idiot would build them on the surface and launch all that mass of people and supplies in giant spheres when building in orbit and shuttling them up would be massively more efficient (although a space elevator would be better - but Bungie already did one of those in ODST).
But, hey, space magic.
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Yeah. They did have one cargo accelerator on the Moon though, which could probably have been used to supply the Colonies in the Solar System. Must have had some sort of setup on Earth as well to be able to get all the material to the Moon, in order to build that cargo accelerator. Blasting stuff off the ground and escaping Earth's gravitational pull is greatly inefficient with the technology we even have today. Must have taken decades to build what we now see on the Moon. Or everyone on Earth started working for free towards a greater good..