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.
I don’t know if offsetting non-capital income with capital losses should be allowed. My intuition says no, but who knows.
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.