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.
77% of Google is owned by mutual funds & other institutional investors.
62% of Apple is owned my mutual funds & other institutional investors.
79% of Facebook is owned by mutual funds & other institutional investors.