Why clothes can smell fine after washing but stale after sitting in a wardrobe

Clothes come out of the wash smelling clean, then a sweater left in the wardrobe for a month develops a stale odor. The cupboard isn't visibly damp, and other fabrics seem fine. Could leftover detergent, body oils or low airflow be the cause even if the item was washed properly? I'm curious what actually produces that ‘stored clothes’ smell and whether airing the wardrobe does much.