Being an external auditor is a privilege, earned through education, testing and the constant meeting of auditing and accounting standards. Audits must comply with various governmental agency requirements (state and federal). To remain qualified, auditing firms spend significant hours and money staying up to date with changes in the profession, on software and research material and on having their audit work reviewed by a third party and occasionally a fourth party.
Additionally, the work spent preparing and performing an audit to meet the standards discussed above is extremely time consuming. Considerable time is spent setting up the audit and determining the risk factors and what areas need additional focus. There are dozens of checklists of information that must be addressed and volumes of workpapers to be created. Anything irregular must be investigated. There is a partner review and a second partner review of the work. A good CPA firm will not certify financial statements without doing its homework.