I have a house. I say have because while I have the title to the house, the bank has a lien that basically means they own it. Like a stock, my house increases in value. The government in my state then taxes me on the value of the house. Taxing me on unrealized gains in the house (I have not sold it) is like taxing a rich person on the unrealized gains of stock (that they have not sold).

It is possible to come up with ways to tax stock. It will be imperfect like all tax systems are. It will be better than what we have now.