Plus, the Big 4 are, by law (SOX), forbidden from doing consulting work. Also, there are for, because public companies need someone to audit the books and someone to help them to prepare those books. Companies have to change auditors and the ones helping to prepare the books ever so often, and you cannot just stick with the same two simply switching chairs. All of that, of course, to avoid another Enron like disaster. Hence a pool of four big accounting firms most public companies, regardless if the fall under SOX or not, cycle through.
