When firms produce products, they pay households for labor. Later households use the earned money to purchase those same products... but since households purchasing ability is equal to firms' expenditure on labor, how can firms earn more than they spend and make monetary profits? It's like imagine one firm - one person economy. Firm pays a person 100 dollars each month for his labor, and produce products... later firm sells those products to that person, but he only has 100 dollars to spend. So it's 100 dollars going back and forth, but no monetary profit is possible.
I discuss it more here: