Most likely, the vast majority are not in circulation but stored somewhere and are outside the US anyway. A stack of $100 bills is probably the best long-term investment for illegal funds that will be used at some point in the future. It's relatively easy to transport, valid until eternity (other than e.g. UK where bills are taken out of circulation every few years or decades) and accepted world wide.
As an example, 80% of the bills in circulation in the US are 100$ bills. It's so much cash that "The average person would be having to carry around 35 or 36 of them in their house or in their wallet. This is for every man, woman, and child."
And according to the federal reserve, the demand for large bills comes mostly from border states - Florida, California and Texas.
Source: https://freakonomics.com/podcast/still-using-cash/