Why is an audit usually done by an independent auditing company instead of the financial managers themselves?

A.j. was a newly hired attorney for idle time gaming, inc. even though he reported directly to the president of the company, a.j. noticed that the president always had time to converse with the director of sales, calling on him to get a pulse on legal/regulatory issues that, as the company attorney, a.j. could have probably handled. a.j. also noted that the hr manager’s administrative assistant was the go-to person for a number of things that would make life easier at work. a.j. was recognizing the culture at idle time gaming.