If it's a small machine with a relatively narrow table, you can build a "C" shaped column mill from available steel shapes and get acceptable rigidity. But as it gets larger, or the overhang increases, it starts needing massive castings for rigidity. A bridge design with a moving table can span a larger area widthwise, since the spindle's not out on a canteliever, It's an easier kind to build if you don't have a foundry, but eventually the twisting moment from an extended Z axis with the spindle on the end gets harder to constrain as its length increases.