Because we don't have a global minimum wealth tax.
Countries have shown that they can work together to implement a global minimum corporation tax[0] to prevent the race-to-the-bottom / tragedy-of-the-commons dynamics of corporations playing countries off against each other, so why can't countries now do the same for taxes on the wealth of ultra-millionaires?
I would base it on Elizabeth Warren's plan[1] for a national wealth tax in the US, which would only apply to the top 0.1% of households there.
[0] https://en.wikipedia.org/wiki/Global_minimum_corporate_tax_r...