https://coppermind.net/wiki/Spheres#Value
Sapphire and Amethyst are tied for second-most valuable, after Emerald. Emerald makes food, and Amethyst makes metal. Very useful. But Sapphire makes air, and only if air Soulcasters even exist, which I'm not sure they do.
Lacking any other explanation, I'd guess the air Soulcasters exist and are used widely for sanitation. Strange that we've seen smoke multiple times though, and never air.