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.
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.