Flat lifter cams are typically machined from a nodular cast iron blank that has the cam bearing areas and valve lifter travel areas cast in the rough. There is still much finish machining to do, but the as cast part at least looks like a cam.
Roller lifter cams start as a forged steel round blank that is completely cylindrical across its length. There is much additional machining just to get to the rough machined cam shape of a non machined cast cam.
There are also material cost differences. Pound for pound, cast nodular iron is less expensive to produce than forged steel.
