Because the supply of capital is kept restricted to ensure that there is not an excess of demand for labor. Do you think the ruling classes are idiots?

When labor was given control of the central banks after the Great Depression we had 40 years of massive growth in productivity and wages. Once the capitalists took back control in the 1970s (after the threat of expropriation had receded) they quickly got things back on track and ever since wages have stagnated while profits have soared.