People can avoid tax with fiat currency too. If you transact everything in cash dollars, and keep it out of regulated bank accounts, you can "avoid" tax.
But the risk is the same with crypto as it is with regular cash. There are ways that the IRS can discover taxable activity and there are penalties that can vary based on the behavior of the taxpayer. When the taxpayer is deliberately trying to hide taxable activity from the IRS, those penalties become most severe.