There are a large number of laws that create a paper trail when substantial amounts of cash are used.

If you withdraw more than $10,000 in cash, your bank has to report it to the government.

If the bank thinks you're 'suspicious', they have to report it to the government.

If a business accepts more than $10,000 in cash it has to report you to the government.

The US is actually more lenient in this regard than some countries, where large cash purchases are (or will soon be) banned outright.