That's true, but most of those can be cracked down on simply by saying that any undeclared wealth is forfeit. Also, the great proportion of most rich people's actual wealth is in forms that are easier to trace (e.g., shares of corporations, real estate).
There is no country where a person has to declare all their possessions or they are otherwise forfeit. That is transparently bad policy. Possessions are one important basis of wealth.
This is, I think, another example of people's intuitions about tracking wealth just not being very robust.
