I think they're a few big reasons for that. Insert usual caveat about how I have nothing but contempt for the absentee figurehead shithead-in-chief, and my praise here is strictly for the actual engineers, technicians, and scientists doing the real work. Or to put it another way: they're not really Elon's rockets!

First, SpaceX is immune to Wall Street shenanigans because they're not publicly traded and have no debt. They're a private company that's been profitable most of its existence. There's no share price to screw with, no loans to use as leverage over management, no way to do a hostile takeover.

Second, SpaceX is extremely vertically integrated. They make most of their own stuff. They very rarely outsource anything. When they do, it's more like a "job interview" than a long-term arrangement. If the company they outsource to doesn't deliver, they're never used again. If they do deliver, SpaceX buys them. SpaceX also has an extremely advanced and centralized system for managing all aspects of all projects. Imagine Allende's Project Cybersyn but just for aerospace. Does Boeing even make its own jetliner wings anymore?

Third, right now SpaceX lives and dies by the reliability of its Falcon 9 rocket, especially the reused first stages. Without Falcon 9, they'd be dead in the water, no payload going up, no customer cash coming in, no resources available for Starship R&D. As part of the cost-savings reusability, they need to do a huge amount of QA on technical aspects that a lot of other rocket manufacturers might not care about recording for a disposable rocket.