I get the feeling that Elon is a pretty controlling micro-manager and has specific opinions about every process he touches that either wouldn't fly with a subcontractor, or he doesn't trust a subcontractor would change if they thought it would change. Those opinions are likely formed and then not worked on (by him), and are rather spread out over the entire process in a way that means Elon's opinions can't be contained to just one department.

(also, I feel like I'm a weird rare person who is involved in both psychology and mechanical/software engineering)