What do you think should happen to you if your house is more valuable in a year than the year before, even if you aren't selling or otherwise leaving that house?
It seems quite reasonable that unrealized capital gains would be treated differently for "a primary residence" vs "a multi-billion-dollar stake in a company controlled by the owner."
A far better question is: Why does my company pay me in cash (40% marginal tax rate) instead of "equity shares of 'special partnership units' representing the value added by verteu's labor" (20% capital gains tax)?
Or: "How did Mitt Romney's Roth IRA grow to $100,000,000 with a $7,000 annual contribution limit?"
I imagine there'd be some net worth number, excluding retirement accounts, that policy wonks could work up. You draw the line between "wealthy" and "regular" there. Or, more likely, several lines because there would be wealth brackets similar to income brackets. Without that it would be a regressive tax.
I'm actually against property taxes, or any kind of tax where you risk losing property just because you managed to live another year.