Like that the market value of an asset can fluctuate wildly and become untethered from the actual rational value of the asset.
So imagine you're Bezos or Musk and you're not trying to be super rich per se but just trying to build a company according to your vision and so you have a bunch of equity in the company (so that you can control it) but no matter how loudly you proclaim that the market is overvaluing your stock the price keeps going up and now you have to sell lots of shares in order to pay the wealth tax assessed against your equity and you don't have a controlling vote anymore and your vision for your company just dies.
And maybe the hordes of people who think no single person should have a controlling share of a billion-dollar company are right - I don't know. But I'm pretty sure a large fraction of people who build companies (like Bezos or Musk) are doing it to build an enterprise, and not for the cash. Otherwise they'd just stop and have fun with their private jets at a certain point. So if a wealth tax were not going to let them keep control of their enterprise beyond the first decade, they probably would opt not to waste their time building it at all.
And maybe that's fine, but I'm not sure what society looks like in a world where people don't want to build companies.