The thing is, the value of people's labour is variable. Simply put, some people do the job more efficiently or do it better. And how do you incentivize skill acquisition? Engineers and doctors spend considerably more time learning their trade, carry more responsibility for the products of their labour, and generally work harder per hour than a sales clerk. The same goes for manual labourers, who literally destroy their bodies while a secretary doesn't face these physical demands no matter how hard they toil. Somehow you need to add value to labour, and money is one way of doing that.