If you step back a bit, and look at the problem more broadly, it becomes clear that a major cause is the current level of consolidation, across the board.
In particular, just in your comment, you name the planemakers—Airbus and Boeing—and the "independent" auditors.
If there were two dozen different companies manufacturing planes, all of which were required to comply with the same regulations, and two dozen different major auditing companies, the chances of any given manufacturer getting away with this level of illegal corner-cutting for any length of time drop precipitously. At the same time, the costs of doing so rise, as it becomes much, much easier for the US government and various airlines to cut a manufacturer out of their contracts the next time they come up.
In particular, just in your comment, you name the planemakers—Airbus and Boeing—and the "independent" auditors.
If there were two dozen different companies manufacturing planes, all of which were required to comply with the same regulations, and two dozen different major auditing companies, the chances of any given manufacturer getting away with this level of illegal corner-cutting for any length of time drop precipitously. At the same time, the costs of doing so rise, as it becomes much, much easier for the US government and various airlines to cut a manufacturer out of their contracts the next time they come up.