Your premise is incorrect and historically ignorant. Slaves are net more costly and drastically less efficient than free employees. George Washington, among numerous other slave owners of the time, for example wrote about that fact indepth. The net cost of workers skyrockets and their productivity implodes, in the case of slavery. Washington made the economic point that with slaves, he became entirely responsible for the cost of their existence, in all regards (shelter, food, health, etc), and their work output was at best on par with or below that of free labor (which is exactly what one would expect). As a system, slavery was a failure morally and economically.