Is it that many people work in areas where it doesn't really matters if they are mistaken? Or do people already know enough about the area they work in and further improvements have diminishing returns?

More the latter. Most of the things that a person could learn about are things that won't help them directly. Agreed that if one has poor epistemic rationality, it's hard to do the instrumental rationality part correctly ("I know, I'll fix this problem by wishing!").