It sort of makes sense to me with SpaceX. They’re presumably buying fairly boutique parts that likely already require custom manufacturing, so someone is spending capital either way. I can see how it might make sense for them to build a custom manufacturing line instead of paying someone else.
That seems odd for commodities like titanium, though. Even if Boeing were to do it themselves, that oversight process is already a subset of the mining and refining process. They’re going to have to build out their QA lab either way.
