Because the agencies KNOW how these crimes are committed and who is committing them but they just don't have the money and personnel to actively pursue these cases and make them stick (so no one weasels out on minutiae of the law after they are convicted).
I read a stat saying when the IRS pursues rich tax cheats, they get a return on investment of about $4500 per agent hour. I don't know what an IRS agent makes an hour, but that seems like an incredible ROI. How many social security payments would that be, school lunches etc per hour.