... Because of the Judges. This is the inside of their planet. The outside is completely scorched and dead, and the inside is finally breaking down too. When we struggle in the games, the -- energy, or whatever, keeps them alive. It acts as "tribute."
In exchange for all that, we get the chance to "win" - they grant a wish and send us home. It can be anything, like a power or one person's safety, or the outcome of a whole war.