1) the world is global, if the US did this then alot of billionaires would suddenly move to Canada. Until the tax code is unified people will chose where to live partially based on taxes.
Look at how many people move from Silicon valley to Texas or Washington just before their companies IPO.
People will just move to avoid the tax. THere are plenty of countries worth living in that don't have a wealth tax. France found out the hard way.
https://www.investorschronicle.co.uk/education/2021/02/11/le...
2) Wealth is not as easily measured as people want it to be. And most wealthy put assets inside corporations so not much wealth is held in their name. Can you tax a person for wealth held in a corporation outside of the US?
Tax treaties tend to distinguish between hard asset held in a country, factories and real estate and less tangible assets, like stocks, dividends, etc.
3) Right now alot of people complain about being taxed on stock options for illiquid options to begin with. We've all heard horror stories about tech employees having to exercise their options and pay tax on that when they can't actually cash out the shares to help pay the tax burden.
How do we tax an employee who holds millions of dollars in illiquid options that are well in the money? And how is that different than taxing someone on the price of a Picasso painting that a billionaire owns but has very little liquidity?
If you claim that the billionaire can just sell the painting for what people will pay for it even if its pennies on the dollar, then you can argue so can the tech employee. Someone will always buy your shares for pennies on the dollar if you don't want ot pay the wealth tax.