If you tax someone like Musk or Bezos at 90% of their earnings they'll still have billions left in take-home income. Many billionaires get a lot of their income through investments, which are taxed but at a lesser rate than income from work.
Even if the government wants to downsize to the point of being able to run on only sales tax and tariffs, it can't do that overnight because millions of American citizens depend on federal and state programs for crucial. Getting the same services from a private institution would likely be less affordable, even if it is higher quality.
Also the military requires a lot of spending to maintain and develop. We need a strong military to protect the country, and decreasing taxes would lessen their budget and their ability to do that.
But to your point, overtaxing could cause brain drain, where highly successful Americans and their businesses leave the country to another country that interferes less with their financial desires.