Even in the rosiest perception of capitalism, the employer is always in a superior position to the employee. Now, this relationship may leave the employee well-compensated, but the inequality is there regardless. Why is that? The main goal for anyone in a capitalist society is to be the employer of multiple employees whose labor you monetize for your greater profit and their lesser paycheck. However, turning a greater profit requires your employees to be in an inferior position to you. This is since two parties of equal standing negotiating in good faith can never come to an agreement which doesn't also benefit them equally. For example, I would never pay $1000 to a baker for a loaf of bread. However, if I was on the brink of starvation (a subservient position), I certainly would. The terms of the agreement hadn't changed, only our comparative standings. Under this principle, greater profits for the employer are impossible if the employee is of equal standing, since the employee would demand that 100% of their labor's value be issued to their paycheck, leaving the remaining income to be used to pay for business expenses and an equally sized paycheck for the employer. If the business' income increased or its expenses decreased, the employee would demand a raise to reflect the increased net income which their labor provided, and negotiation would doubtlessly end in the employer getting an equally sized raise. As you can see, in this situation, there is no way for the employer to profit more than the employee, meaning that said employer has failed at capitalism. But just like with my bread example, change their comparative standings and the entire situation changes. In a state of equality, the employer can't offer less than the value of the employee's labor and the employee can't request more. If either did so, the other would be in an equal position to refuse. If they don't come to an agreement, they each lose out on an equally sized source of income, which would be equally detrimental to them. If they do come to an agreement, they each gain on an equally sized source of income, which would be equally beneficial to them. In a state of inequality, however, the employer can offer less than the value of the employee's labor since losing out on the source of income is more detrimental to the employee and gaining it would be more beneficial to the employee. Both parties know that the best choice for the employee is for them to take the unfair deal, which the employee does almost always. It is on this inequality that capitalism thrives and it is why a capitalist society will never achieve equality. You can figure this out since the very idea of the employer and employee having equal profits in our culture leads to fears of Marxist totalitarianism.
tl;dr: Since capitalist employers are in a superior position to their employees, they can force employees to accept paychecks which are representative of less than their labor's value, keeping equality from ever prevailing