Die harvesting is not perfect. If there is an error in a logic transistor, they can turn off that cpu core and use the rest of the chip. If there is a short bet

Die harvesting is not perfect. If there is an error in a logic transistor, they can turn off that cpu core and use the rest of the chip. If there is a short between ground and power, the whole chip is useless because it will melt when you plug it in. Yields on larger chips are still substantially worse than smaller chips, even with harvesting.