Someone forgot to model growth in the value of the asset, and/or putting the wealth to use. A wealth tax is, to an approximation, the equivalent of the "management fee" that an ETF charges, but with the revenues going to the government. If you have a bucket of money that isn't doing anything, then what value does it actually bring to the economy? Penalizing static value seems almost reasonable.
We already do that via inflation. Leave your money uninvested and we tax it 2% or more per year, every year.