that's been going on for decades. From the electrician doing side work on the weekend to the cash-paid bartender at the local saloon - it's prevalent. People do it to avoid the extra paper work of reporting income, they do it to avoid going over the "earned-income-credit" level, they do it to maintain government support checks such as disability, unemployment, or welfare bennies.
And the employer benefits as well - businesses running on the edge can stay in business by not having to pay withholding taxes for "undocumented" employees, not to mention increased insurance and government costs that are saved when the employee isn't legit.
Walk through any large city neighborhood and note all the small restaurants, bars, and shops. Odds are that half of them employ somebody "under-the-table".
The quantity of such under the table business is difficult by definition to ascertain. If the IRS claims that one out of six people has at some time in their life gotten income dealt under the table, that might be believable. But that many people doing it all the time? Pulling my leg.