Why do special inspection institutes generally only recruit supernumerary personnel?

According to the relevant information, the special inspection institute generally only recruits supernumerary personnel because of the needs of the post. The supernumerary personnel of the Special Inspection Institute are mainly engaged in the inspection and testing, technical service and inspection technology research of special equipment such as boilers, pressure vessels, pressure pipes, elevators, lifting machinery, motor vehicles and amusement facilities in the factory, and need to have relevant qualification certificates, while the internal personnel generally do not, so they all recruit supernumerary personnel.