Earlier quoted context omitted.
Only because of how we've chosen to define income.
The proposal in the article is to tax wealth at an annual rate of 2%. Not realized gains. Not unrealized gains. Wealth. Under such a system — unless I've badly misunderstood something — if a billionaire's assets decreased in value over the course of a year, they would still pay 2% on their assets. I can't think of any sense in which a decrease in the value of one's assets would be defined as income. I have an opinion…
The "billionaire's tax" Biden proposed earlier this year is closer an income tax that also includes unrealized gains and only if there are tens of millions in unrealized gains in that year.
That said: you pay property tax even if the value of your home declines. It's not that crazy.