I have a hard time believing that Gulf Countries (Saudi, UAE, Kuwait, etc.) haven't surpassed that by now honestly. Is this record current?
As far as I know it's because Death Valley is lower than sea level. It's a giant depression with winds blocked on all sides. The heat gets trapped from the surrounding high ground. There are not enough plants to create shade. No to low humidity means no clouds to block the sun. No or low winds mean no convection cooling. So it just gets hotter and hotter and hotter.