Richard Branson was a billionaire when he started Virgin Galactic, and it was considered a vanity project at the time.
Paul Allen made his billions at Microsoft, then put his money into things like the Allen Telescope Array, air launched rockets, and billions into advanced medicine. A group of billionaires are funding a multi-billion dollar institute into longevity research.
SpaceX was initially funded with the money Musk earned from selling his interest in Paypal. If there were 90% tax rates that were enforced, he would have had no capital to start SpaceX. Furthermore, SpaceX is still largely funded by billionaires - spaceX just raised 1.9 billion for Starship and Starlink development in a private funding round. Since SpaceX isn’t public, that money came from a small group of very wealthy investors.
Actually, it’s more likely that if the U.S. had those kinds of tax rates, Musk would have stayed in Canada. Or at least Musk would have created Paypal in a more tax-friendly regime. Maybe today SpaceX would be in Costa Rica or Indonesia or somewhere else near the equator where taxes were reasonable.
As for all those guys who were’t billionaires when they founded their companies… All of them raised money from other very wealthy people to finance their ventures. No rich people, no financing.
Also, if tax rates went sky-high on the wealthy, what incentive would any of them have to invest? If the investment fails, you lose it all. If it succeeds, you pay 90% of the proceeds to the government. Do you know what those incentives would do to any investment that carried any risk at all? Would YOU invest your life savings in a business that might fail, and if it succeeds you will only be able to profit mildly from it before the government starts taking the rest?
High taxes on wealth and income destroy investments that have a risk element (almost all), especially at the small business level where people are taking one shot at a business and have no other losses to offset profits.
Let’s say there are no taxes, and I have a new for a business that might fail 90% of the time, but if it succeeds I’ll make ten times my investment. If I invest 100,000 in such an investment, my expectation is an overall profit of $100,000, albeit with huge risk.
Now add a tax of, say, 50% on sale of the business. Now, if my 9-1 shot comes in I will make $500,000 instead of 1 million, so my expectation for this investment is now -400,000, and I would be an idiot to invest.
Big institutional investors aren’t hurt too badly, because if they make this investment 10 times, they can offset the 9 losses against the win, so they just lower their profit from $100,000 to $50,000. But if yiu are investing your own money in a small business and have one shot at it, the tax will drive you away. And the higher marginal taxes go, the worse effect on investment it has. Money starts going to ‘sure things’ like government bonds and t-bills instead of investing in an idea that has big upside for everyone, but risk for the investor.