Not literally sitting on them, but they are certainly not spending anywhere near as much of their money as someone who makes ~1M$ a year. A large part of their wealth is usually tied up in long term investments such as property or bonds, where it is safe but not productive.
And as I said, enforcement is a real problem, capital flight being a possible consequence. I don't know exactly what the solution is, but surely there is some better balance to be struck than today. Especially if the US government wanted to - they have enough financial and diplomatic power to enforce this type of tax almost world wide, even if California or France don't.
And for renunciation of citizenship, there's a one time liquidation tax where you pay taxes on all of your gains (so Bezos would have to pay a flat ~20% on his Amazon holdings to renounce). So the U.S. is the only country in the world that could get away with a global wealth tax on their citizens (they already tax Americans' incomes irrespective of country).
I don’t see in what universe a wealth tax could be effective, given that people can simply move. Wouldn’t it just cause large-scale capital flight, as it did in France?