Because many of the wealthiest individuals de facto realize their gains in the form of collateral for loans that allow them to live without ever realizing their gains from the perspective of the tax code. If you do well enough, you
never have to pay taxes on stock holdings.
The very first sentence of TFA gives it away:
Recently, my story as a Norwegian entrepreneur facing an unrealized gains wealth tax bill many times higher than my net income went viral
Many who are in the position to control how their compensation is doled out (board members, C-level) will often take it exclusively (or nearly so) in stock, specifically so they never have to pay taxes on it. Famously, several have taken $1/year incomes - e.g., Mark Zuckerberg and Steve Jobs, while Elon Musk didn't even bother with the charade and took $0/year.
* Side bonus: in the US, corporations paying out performance-based compensation like stock get additional tax breaks, so it's not just the executives which win the taxation game while doing this.