Wet-aging is a relatively recent technique that developed along with advances in plastics and refrigeration. In this process, cuts of beef are vacuum-sealed in plastic and shipped to the market. The aging takes place in the 4-10 days between slaughter and sale while the meat is in transit.
The enzymes still have time to tenderize the meat enough to make it acceptable, and the biggest plus is that there's no weight-loss in the meat due to dehydration. Wet-aging also costs less for the manufacturer since the meat doesn't need to be stored or monitored, ultimately resulting in a lower consumer cost.