On a national basis, the top 1% in the US don't have enough money to finance bespoke art heists (nor can they generally afford black market museum pieces). So it's probably not them :)...maybe the top 0.1% or 0.01% could.

Whenever I think about this particular topic, I come to the conclusion that there must be much more nuance to it. I assume the pieces are eventually sold outside of the United States, maybe to oligarchs or royalty in other countries. Or maybe they end up becoming more like "gray market" pieces that can be semi-safely purchased under the guise of spontaneously "re-surfacing" after a number of years.