I'm not sure there is the massive "losses from unreported cash wages" you seem to imply there is.
There's two main forms of "cash wages" in the US. One being wait staff, the other being "under the counter pay" to workers, generally labor intensive. Wait staff who regularly under report their tips are easily identified by the IRS. In the second case, these generally aren't well paid jobs (thus wouldn't generate much, if any, tax revenue).