The surplus labor army is useful for driving down the wages of the employed, to increase the profits of the employers. Why pay 3/4 of the population to work 40

The surplus labor army is useful for driving down the wages of the employed, to increase the profits of the employers. Why pay 3/4 of the population to work 40 hours a week when you can pay 3/4 of the population to work 40 hours for what they used to be paid for 39 hours, holding the threat of the competing jobless 1/4 against them? This is grossly simplified, of course, but why would any company try to optimize for the benefit of society over their own?