I understand the difference between stocks and flows.
Do you consider human capital a part of personal wealth? It’s reasonable to do so, but how do you properly measure it? If you have income from a risky source (for example, dividends from high-beta equity shares), what interest rate would you use to capitalize it to a present value?
Most people can calculate their current annual income with the help of a few documents (pay stubs, bank statements, etc.) Very few, and certainly not the government, can calculate a comprehensive, reasonable accurate measure of their wealth.
I used to ask my students if they were wealthy. Most would reply something like, “No; I only have a part-time job, a ten-year-old car, and some junky furniture.” But they didn’t understand the distinction between temporary (current) and permanent (life-cycle) wealth. A sensible wealth tax would, at a minimum, recognize that distinction.
There is a large professional literature on wealth versus income taxation that deals with both the theoretical and practical difficulties of properly measuring the relevant base.