People are holding Tether and don't realize it. Most BTC/USD exchanges aren't BTC/USD exchanges, they're BTC/USDT exchanges, but that "T" is stuffed behind an asterisk. People that want to make profits don't realize they're not actually cashing out for $10k, they're cashing out for 10k USDT.
As soon as they wired USD to one of these exchanges, they were stuck holding USDT. They could take the BTC and run to another exchange like Kraken, but that has transaction fees associated with it and they don't want to lose ~10% of their investment.