You are correct the IRS doesn't but the US government does. This would fall under suspicious activity. "Under the Bank Secrecy Act (BSA), financial institution

You are correct the IRS doesn't but the US government does. This would fall under suspicious activity. "Under the Bank Secrecy Act (BSA), financial institutions are required to assist U.S. government agencies in detecting and preventing money laundering, such as: Keep records of cash purchases of negotiable instruments, File reports of cash transactions exceeding $10,000 (daily aggregate amount), and Report suspicious activity that might signal criminal activity (e.g., money laundering, tax evasion)"