Cause they get paid in straight cash homey.
Sometimes, but not as often as people would think.
Attorneys who make lots of money typically have prominent clients. Prominent clients are more often than not business entities. Business entities want to be able to take deductions for their expenses. Payments for legal services rendered to business entities can often be deducted. It doesn't behoove businesses to pay attorneys cash under the table, because then they have no way of deducting those expenses.
Same thing goes for large firms who have associates and junior partners which make $100,000+ a year. The wages they pay their employees are deductible. Pay them in cash and you can't deduct it. Not to mention that I've never seen a large firm handle their payroll in cash.
I don't doubt that many attorneys receive cash and don't report it, but I don't think it happens quite as often and in as large amounts as many people think. I highly doubt that the 27% of full-time attorneys who reported making less than $50,000 a year before taxes have tons of cash payments that they're hiding which would significantly increase their income.