Because they are American taxpayers.
How are the real taxpaying Americans supposed to hold them accountable? Where is the oversight board looking to ensure essential social services stay available? It all seems like a recipe for disaster - love them or hate them, Elon and Trump have obvious conflicts of interest that make this process disturbing.
Spending has been "unsustainable" for 40+ years, ever since Reagan gave up on trying to get his own party to reduce it. I'd say that's a pretty good indication that the status quo was working well enough.
Musk and Trump are not concerned about debt or spending. If they were, Trump and Republicans wouldn't be rushing a billionaire tax cut bill that will add tens of trillions of dollars to the national debt.
If Trump cared about debt he wouldn't have run up $8 trillion in debt his last term with PPP helicopter money and more billionaire tax cuts.
Or proposing to create a PIF-like Sovereign Wealth Fund while gutting USAID, an organization with approved budget for many many years that actually provides assistance efforts globally.
Or proposing to invest $500B on AI development, which, coincidentally, just so happens to be the amount Sam Altman asked for to develop Stargate.