In a company with failing engineering quality, what typically happens is that, of all the engineers qualified to sign off, management over time tend to choose the ones who are more willing to sign off. The more careful engineers who refuse to sign off, or attempt to negotiate harder for safety before signing off, are not promoted over the less careful engineers. The result is that the engineers in a position to sign off increasingly comprise the less careful ones.
Who is to blame in this case: the management who drive the process that selects the less careful engineers, or the less careful engineers?
The leadership (including the board who are proxies for stockholders) is ultimately liable - they hire, fire, set the strategy, values, and goals for the company. They reap the rewards of profits and take on some risk. That's the difference between a regular employee and an officer. When you have a business that involves public safety like car and airplane manufacturers, a large part of your business should be ensuring that everyone from the lowest level employee to the CEO is at or above industry standards for safety.