You're thinking that taxing the wealth of a billionaire is guaranteed upon death. Therefore from your perspective there are two options: taxing the wealth or not taxing the wealth. Except taxation is the exception, not the rule.

If you know your wealth will disappear then you will try to get rid of it before it can disappear. With the estate tax there is a strong incentive to just hand over your wealth to your children before you die or by spending it all on donations so the government can't take it.