Except billionaires take advantage of a loophole whereby they can take out loans using stock as collateral, instead of selling the stock, meaning they pay no capital gains.
If we taxed land, it would be essentially impossible to evade, even with fancy accounting and shell companies and all that stuff the rich love to employ. All this is exactly why the how of taxing the rich is so important.
Sorry, I'm a traditionalist on this matter. What you do is take 20 strong men-at-arms and force them at swordpoint to show you where they've hidden their valuables. 100% tax.