Employees are more expensive because they are less replaceable. A company must invest a certain amount of money into labor to make a profit; however, if that company learns it can invest less money into endeavours to make the same profit, then it can decrease the amount invested into labor. The only way to do so is to create some sort of technology, or social relation, that makes the price of individual workers cheaper. Thus, any reduction of cost of labor that increases profit is something that makes employees more replaceable.