A battlefield is a fluid situation. A rifleman can move around and seek to find a way past a fixed machine gun position. Even a machine gun nest can be suppressed by return fire, and troops on the move can have an advantage if they have semi-auto vs. an enemy with a bolt-action.
Sure, the MG42 was a helluva fine piece. Still, absent opening up as ambush, it's firing from a fixed postion and a fixed position is best known as "target for artillery".