Banks and businesses are required by federal law to report cash transactions over $10,000 to the government.
Under the Bank Secrecy Act, banks and other financial institutions must file a Currency Transaction Report (CTR) with the Financial Crimes Enforcement Network (FinCEN) for any single cash deposit, withdrawal, or other transaction exceeding $10,000.
They are also required to report any suspicious activity, regardless of the amount, including attempts to circumvent the reporting threshold by breaking up large transactions into smaller ones (a practice known as "structuring"), which is illegal. FinCEN may then share this information with other agencies, including the IRS, FBI, and DEA, if they suspect illegal activity.
The main purpose isn't to detect tax fraud, but there is opportunity for the IRS to discover it and question.
When I do stock transfers to somebody else's account, ETRADE classifies it as a 'gift'. Not sure where the reporting goes.