The issue is figuring out which assets they have and are they the true owner. If you have assets distributed across dozens of banks and other parties who are holding them for you, this becomes incredibly difficult and rife for abuse, especially when you have other non-professional/non-traditional intermediaries holding assets for you, or for whom you are exchanging favors with.

Basically compiling the list of the assets they hold and verifying it is the biggest issue here. It is definitely not at all simple -- in fact it may be deliberately obfuscated for "reasons."