"The Federal Reserve and Treasury make 99 dollars for every $100 dollar bill they print and sell offshore," Colas said. "There's a natural desire to keep printing these things — the U.S. government makes a lot of money selling them."

https://www.cnbc.com/amp/2019/02/27/theres-been-a-mysterious-surge-in-100-bills-in-circulation-possibly-linked-to-global-corruption.html

Pretty good read.