" why didn't the factory use them instead of torque boxes that tie the sub-frames to the rocker boxes? You see a lot of factory installed (on all makes) torque boxes on high H/P cars and convertibles, but rarely (if ever) do you see factory installed sub-frame connectors"

The factory relied on the rockers mainly, the convertables had a 1/4 inch steel box tube inside the rockers with torque boxs tied in for strength. But controled flexing kept these cars from breaking up to last 100,000 miles in normal driving. But if hammered on, with todays roads, connecters are needed