Why does the car wash industry pay less social security?

Small organization, short working hours for employees. The car wash industry is mostly private car wash shops, and medium and large private car wash shops will pay social security for their employees. However, in small institutions such as ordinary private car wash shops or small private car wash shops, most of them will not pay social security for their employees, and the employees in the car wash industry have great mobility, short working hours and little social security.