That works for real estate, and indeed property taxes are a long running staple of government taxes precisely because land is fixed. Economically, land value taxes are considered the most efficient form of taxation as they have no dead-weight losses, and were championed by Ricardo. But if I have a contract that assigns 10% of the earnings of some mutual fund to a holding company in the Bahamas, then it is not so easy to convince the government of the Bahamas to join you in seizing the asset. The money is overseas, and the wealthy only need to transfer to your jurisdiction whatever they need to spend, which is generally much, much less than what they earn in a given year.

So what happens when you try to tax unrealized capital instead of land is that the former is highly mobile and can arrange to be not subject to your jurisdiction if the rate of taxation is higher than whatever inconvenience is obtained by moving the claims around, so the net result is a loss of overall tax revenue. This is why France abandoned the ISF and replaced it with a tax on land, which is not mobile, and why most jurisdictions have low taxes on capital compared to less mobile things. It's not because it is "fair", but because there are limits to what you can collect that don't apply to things like land or labor.