I wish this were true, but it is not, trust me, some of us have tried 

While many people do use "cash under the table" to successfully outsmart the authorities, many more are caught and convicted.

This is because the IRS set up a "whistle blower program" (AKA, bounty hunter program).  Essentially if you see an employer peeling out Benjamins to an employee and not reporting it, you can turn his ass in and get a 15-30% cut of the loot when the IRS raids his business .

This has made tax evasion using "cash" extremely risky.  With bitcoin, all the Whsitle-blower can report is some furtive typing on cell-phone keyboards, or nothing at all.

Also, bitcoin could soon allow encrypted banking (operating an all cash business without a bank is risky), and the IRS monitors bank deposits to verify reported income.

Also remember that unlike Cash, Bitcoin is easily scalable.  The IRS goes after the biggest tax cheats first and then works their way down the chain.  It is awfully hard to conceal millions of dollars of cash income (just ask drug dealers).  It is (theoretically) as easy to conceal a billion dollars worth of bitcoins as it is to conceal the $0.15 I've generated so far (yay!)