In addition to being a highly volatile source of revenue to the government, you would have to give tax breaks for unrealized capital losses out of fairness; which would introduce a ton of complexity.
the US government already effectively does this to foreign resident citizens who buy PFICs (and why us citizens are strongly encouraged not to buy PFICs).
the reason its problematic is that just because a persons assets have risen, doesnt mean they want to sell, and if they dont sell, it means they need the liquid assets to cover said tax bill. a tax of this nature forces people to sell assets which is problematic. Its not problematic to say you sold X assets at profit Y, its your responsibility to retain percentage Z of that to pay the taxes.
