Are you freaking kidding me? The reason for that is explained/implied in the fiction. The Halo rings have a limited power supply, and floating architecture was unnecessary. Same goes for the Ark. But Requiem is/was a shield installation where Forerunners could live and work inside without fear of the Flood getting in. It also acted as a staging area for rapid attack and threat response. As for the explanation for the multiplayer, it's just a nice added touch. It's a reasonable explanation for why Spartans are fighting each other. Without that context, it just seems like mindless mass murder. Placing it inside the context of a combat simulator helps remedy that. And it's not like this is the first time lore has been implemented into multiplayer. Look at Invasion in Reach.