It’s not a conspiracy theory, it is a well documented fact. And they didn’t just steal from the Germans/nazis, lots of priceless and irreplaceable art from all over Europe ended up being shipped to the US.
To quote one of the articles I linked below: “It has famously been said about World War II soldiers that certain countries fought for certain things. Britain fought for Honor, Russia fought for the Motherland, Americans fought for Souvenirs.”
https://www.warhistoryonline.com/instant-articles/who-looting-and-us-troops.html