Manufacture and maintenance contracts for the Shuttle were deliberately spread out across many companies and states, especially in key congressional districts. It was a jobs programme; waste was a feature not a bug.
More fundamentally, there were contracts. SpaceX does things itself; there's no legal friction internally. This gets back to the "Theory of the Firm" for why firms exist in the first place (transaction costs).
The need to codify what work is to be done in contacts is antithetical to SpaceX's rapid development processes.
Same thing for SLS.