I think that institutional arrogance is built into the DoJ. It's one of my bugbears with large organisations, generally. But they have it in spades.
Add to that their cowardice and complicity in the face of Trump, and perhaps you have your explanation of why they don't want a candid investigation. Because they will themselves be exposed, starting (but definitely not ending) with AG Barr.
It may need a whole new department to oversee the DoJ. But in the US, nothing will happen, because the whole machine is dysfunctional.