Boeing doesn't just make planes. It's a huge company, so there is plenty of things to audit. I also expect that the payment terms for planes are extremely complex and may carry hard-to-quantify risks for the company.

"We know that Sarbanes-Oxley was a good effort at tightening up controls."

On the contrary:

Shortly after Sarbanes-Oxley came the great financial crisis, during which numerous financial institutions whose books said they were just fine somehow turned out to be in the hole for hundreds of billions of dollars.

In 2005 November, it became glaringly obvious that many mortgages were not worth $#!%, and the procedures used to cover up this fact were strikingly Enronesque - except that they involved sums of money millions of times larger than Enron.