Why didn't prudential life sign a labor contract?

Not a regular employee of the insurance company.

For example, the salesman is an insurance agent, not a formal employee of an insurance company, and has not signed a labor contract. If it is a foreign recruitment, it is an agency contract, and the contractual relationship is an agency relationship rather than an employment relationship, so a labor contract will not be signed. To be exact, the on-site salesman is equivalent to the agent of the insurance company, not the employee.