it's better for them to check every exchange in the U.S. although many people keep their bitcoins in private wallet.
Most of the people using an exchange never actually withdraw their coins to their own wallet.... the only transfer taking place is exchange to exchange where they trade crypto back and forth until it's time to sell.
It would be a better idea to ask an exchange to be specific about unique users who actually made a fiat deposit to buy Bitcoin. Creating an account can be done by people that were here for years already but want to cash out now.