A business's profit is the excess value created by workers. Low-skill workers don't get the true value of their labor because they have no negotiating leverage in many cases (especially with the downturn in union strength in the US). It's not a symmetric negotiation, as it's easy for the big and powerful company to find other employees, because in many cases the workers' only alternative to being underpaid is being unemployed and homeless. Business owners exploit the basic human need for food and shelter to underpay low-skill workers.