I can definitely see why most quality internal audit checklists are planned by managers and internal auditors. These two groups of people definitely have the expertise for formulate an appropriate checklist.
Managers have extensive knowledge of the day to day working of the company. They know exactly what activities are performed, and they probably know what areas need work. Internal auditors know how to audit and probably know of a few things to look for that managers wouldn't.
Although, I could see this backfiring if the managers were trying to hide something.