Canada

How do you value something that was not sold? Some people can paint paintings worth millions others can paint the same painting and it's worth less than the canvas. If you need to go through and value everything that everyone above a certain threshold has every single year you will get bogged down with accounting work. Especially since these people employ teams of accountants that will be deceiving you every step of the way. It's much easier to do a one time big taxation event than a small recurring one (see Nate Erskine-Smith's discussion on this). It's also much fairer. While you are living you should be rewarded for high earnings, but when you pass your children did not earn that wealth.