Some people do get a sense of pride and fulfillment out of doing a good job that money can't provide.
... is apparently the argument.
I mean, see also: drug tests for the minimum wage workers, but the CEO? Of course not. Mere employees better not be moonlighting or doing anything else, but the CEO? On two other boards, "advising" a startup on the side, working on another part-time (also as CEO—their "executive assistant" at their 20-person startup actually runs the company)
The issue with the 737MAX is that Boeing was too risk-averse: they should have made a brand new plane, but went with what they thought was the low-risk route with the 737 re-engine. The 787 disastrous development ($36B of deferred production costs I think) was certainly a huge cause of this.
if they just stuck to evaluating technical risk first, and only then the risk of selling the plane - perhaps that route would have led them to developing a new completely plane.
But a good job for a CEO is building shareholder value, which is now understood as "[wringing] all the money out of the company that they can [for the shareholders]." That's a different job than building an organization that consistently chooses safety over schedule and cost.