Yeah, it seems like most people assume there is a reach and scope of taxation that isn't really possible. Wealth can be expatriated, it can be in non-fungible objects (paintings, &c), it can be in goods held in common such that no transfers occur (for example, a house that people live in together and jointly own).

There isn't anywhere an index or lookup table of all legal rights a particular person has to wealth (or, in truth, to "things", since anything can be worth something and contribute to wealth). There are things they may have a right to that they don't even know about.

They can hide wealth (at their own risk) but it prevent them from extracting money from the country:

The cannot own houses, factories, monopolistic contracts o media. It makes harder to influence politics ( in a legal way).

The housing issue is specially important because city space is limited and the demand is very inelastic

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).

Money is important as a vector for power. It doesn't matter that much whether a person has a bunch of paintings in a Swiss vault when they're an institutional investor directing a substantial sector of the economy. And that industrial power is relatively easy to divest them of, as compared to vault paintings.