Michel Van Rijn was a 'famous' Dutch art smuggler. He explained that a lot of art was ordered. The extreme wealthy of the underground just buy it because they want it.

A Vice article about him: https://www.vice.com/en_us/article/ex5qj4/how-i-became-one-o...

Edit: the Vice article doesn't mention the thefts and looting. There are other sources for this. It is also unclear if he really is a former smuggler.

Edit 2: The other reason is it's relatively easy to steal art. And most thieves focus on that part. The selling part is difficult and is also why a lot of stolen art is later recovered.