Wealth is how much money you have to spend, not what your theoretical paper worth is. The government can’t buy medicine or fix roads with theoretical paper worth, and until you can buy food with stocks / shares / yachts / art, it shouldn’t be treated as if it is taxable currency.

Taxing the flow of money is the only moral way a government can bring in revenue and the only way to increase that revenue is to raise taxes. Higher income tax and higher capital gains tax means the more you benefit from sharing this society with everyone else, the more responsibility you take for funding it through taxation.