What you may not be aware of is that this curvature decreases as you move from the driver to the fairway woods. The greater the dimension of the clubhead from face to back, the more horizontal curvature is required across the face to make the gear effect perform properly. To state this another way, the farther back the center of gravity of the clubhead is from the face, the more bulge is required across the face to make the gear effect work properly.
Irons are flat-faced (lacking bulge) because their dimension from the face of the clubhead to the back of the clubhead is far less than that on a woodhead. Thus, the distance from the position of its center of gravity to the face of an iron is far less than it is on any woodhead. Therefore, iron faces do not require any bulge, and can be made flat.