Financial assets are extremely easy to hide. Set up an international chain of shell companies, foundations, and trusts, install a fake beneficial owner or trustee or two at various points, carve out deductibles for IP and "services", and the ownership becomes completely opaque.

And that's just the legal version.

I know someone who used to work as a business lawyer. She spent years trying to track down the true owners in various cases. At the very least it's an expensive business. And sometimes it just couldn't be done.

Of course governments can cut the knot with physical assets, walk into a building with troops and/or police, and say "This is ours now." Or they can order banks to hand over the money in accounts.

But before they can do that, there has to be some certainty about the owner. And even getting part way there can take a while and cost a lot.