Earlier quoted context omitted.
Because he owns companies that contract with the government and are affected by its policies. It's the very definition of a conflict of interest. And he's not even "auditing" the government. When you're auditing, you emit a report that the audited party later analyzes and acts upon. He's been given freeway to fire government workers as he pleases as if he's an elected officer, which he's not.
Washington has been known for revolving doors among particular industries for quite a few decades! Why the hoopla over this one? They are auditing as part of their process of cutting costs. They're literally tracing trillions of dollars in financial records. He's doing everything he's doing by executive order of the President of the United States, who was elected.
And already has a number of lawsuits started because he's trying to do things neither he nor the president are allowed to do. Getting an EO to do something doesn't mean it's automatically legal. Multiple big decisions have already been reverted or are held until judges can review them. Even things like the promised payout for quitting are not practical, because only congress can approve the money for that.