I bonds still carry some interest rate risk. If interest rates go up and inflation stays constant, they'll drop. Also, they're indexed to the Consumer Price Index, which measures the cost of food and gasoline and other consumer products, but does not include some other important aspects of the cost of living, most notably housing prices.

After inflation and taxes, you can get about 2% risk free. If you want to retire at 20 and live off the interest of your poker winnings, you'll need $5 million for a consistent $100k/year income.