Large construction sites usually hire unskilled labour from close to construction sites because it's cheaper. They don't have to provide housing for them. Because they're unskilled, they don't have to bear the cost of moving them around with them.
So naturally, they'll recruit at a local recruitment office or ask a contact person for assistance.
Off topic I know but many "government" contracts require that local labour and BEE subcontracts up to certain percentages be used by the main contractor; 30% is typical for subcontractors. The medium sized contractors never pay cash, they have to have a bank account number; and it is a record keeping requirement that all labour (and subcontractors) have tax reference numbers. BUT, for the subcontractors anything goes. We have a case now where the local labour wants to be paid directly by the main contractor as the Level 1 BEE subcontractor is not paying them. Not possible unfortunately (for them).