Assets can be easily hidden by elaborate constructions over multiple nations, if you have the money to finance it you can save millions with it. Is it legal? Maybe yes, maybe no. Is it something desirable to society? No. This is why you need a registry.
What assets?
Land can't be moved, company offices can't be moved.
Just tax what you control and let others tax what they control. Why do you care if I have a pile of gold stored in Barbados. It doesn't influence your country in any way.
