It’s Dark Sun, and therefore “not your father’s underdark”. I am sympathetic to the “isolated pockets” underdark suggested upthread by Jason_Goff. The isolated pockets works better because you don’t have to account for an entire Forgotten Realms type underdark, and you don’t have to explain why people cannot access deep earth ore deposits.
Just because there is life down there doesn’t mean that it’s the kind of lifestyle that most surface dwelling humanoids could survive. The creatures down in the underdark pockets would be adapted to it.