Why would they need to sell anything? Just take a salary large enough to cover the wealth taxes. A CEO with a $100M net worth should easily be able to command a salary greater than $1M.
Some are. We live in an area where annual taxes are about 2% the value of the home. Home values have been skyrocketing and taxes along with them. Some people on fixed incomes can't afford it so are forced to sell. I think property tax should be abolished in general, but the problem with taxing ownership of a company is that owners are then forced to spend that much less on expenses to cover their tax burden (fewer jobs, or lower salaries for employees).
A lot of companies operate close to break-even or even at a loss. What happens with the owner of a $2m / year company must come up with a 10% wealth tax amounting to $200k, but the company is operating at break-even? Expenses must be cut to pay the tax. Ultimately hurts the lower and middle class the most.