Liquid metal printing, developed by MIT researchers, rapidly creates 3D metal structures by depositing molten aluminum along into a bed filled with microscopic glass beads. The process can produce low-resolution objects like frames for chairs and tables in minutes.
With sand casting you still need a mold to form the sand. These molds aren’t easy to make, especially quickly. With this 3d printer, you can go from design to finished product long before the mold is constructed.
There are also things that can be 3d printed that can’t be easily cast, like things with oddly-shaped holes, and hollow objects.
This is the same as asking why 3d print something when injection molding exists.
In terms of speed, you’re totally right. That said, thete are tons of methods of turning a plastic 3D printed part into a metal part via sand casting that you can do at home. With enough money, evidently you can skip the plastic stage completely: https://www.exone.com/en-US/parts-and-services/sand