Chinese habit is to always carry a lot of cash when traveling. Credit cards are not yet popular, and they may not have a foreign bank account back in China to pay it. Credit cards are still not widely accepted in China outside of big cities or larger establishments.
Plus people don’t trust the government there, so they keep a lot of cash in their homes as well.
Years ago, I met a student from a dirt-poor rural part of China - probably the first in his area to go to university. He was on scholarship, which gave him his stipend every 6 months. He had 1 pair of pants, and he literally sewed his scholarship cash into the waistline of his pants - he was that concerned about losing it.
So when he washed his one pair of pants, he was laundering money?