I think the fact is when a person is ultra rich, they can avoid ANY tax system legally.
Let's imagine a wealth tax on just stocks and eqs. Dude has a billion in say Apple and it doubles, and you tax his billion now at 5%, or $50 million. Europe has shown you don't raise much revenue this way, in part because he can leave the country, or convert into some other untaxed asset (which would incur capgains), or just pay it, but that means he'd have to sell a portion to pay the tax, which would put a downward drag on the stock price if multiplied out.
The main point for me is that it doesn't appear to generate enough revenue to be worth it. (And it may be uncon).