Try and introduce that and you'll run right into the impossibility of measuring wealth. E.g. what if Bill Gates just gives all his many over 1 billion to a charity or trust? Say, the Gates foundation (see wikipedia for the good work that charity has done)? Is the wealth "his" given he still has a great deal of control over how it gets spent?

Try a wealth tax like that and the Musk charitable foundation is founded with the goals of fighting climate change via electric cars and making a Mars colony.