It is end result of free market. Every side, consumer, buyer, seller, employer, employee and so on try to get away with paying least they can. This is usually by choice, not many actually combat this... The strange bit is how we have ended up in situation that there is some moral mandate that there is extra payment between consumer and employee. In all other parts system operates rationally. You pay what you have to get something. Be it the product or the labour. Paying more will have indirect benefits like better retention and maybe better quality(more effective or better selling staff).
On employee side, well there is usually not option to forgo work as money is needed for survival... So their only option is to take work that is less than they might think they are worth. But this often results in poor retention as they will move for better options when possible.