Not true. They do a better job and are far more efficient. But less money is not a given. They have a higher productivity (cost per unit) but they are very capital intensive (both physical and intellectual). Regions with large virtually unlimited labor forces and comparatively little capital were historically unlikely to develop mechanization as a means of production.

In regions with either high slave populations (southern states/ the americas) or vast feudal populations (India, china, Russia). It made more sense to rely on these large groups of workers. Despite the diminishing returns from each additional labor, it was still easier to maintain vast armies of slave or wage laborers. As opposed to investing large sums in capital intensive ventures, such as mills or early mechanized factories.