A big part is that CPU sockets are very standardized and you are only cooling the CPU and not the memory and power regulators. For graphics cards the coolers ne

A big part is that CPU sockets are very standardized and you are only cooling the CPU and not the memory and power regulators. For graphics cards the coolers need to cool the gpu along with the memory and power regulators which can be in different places depending on the manufacturer. This usually means you need a block made specifically for your model of card. You can buy those blocks and do custom water cooling but the cost of that is incredibly high. Also it finally comes down to the AIO coolers really aren't better than good air coolers. Sure AIO tend to be quieter and move the heat to a different location but on pure performance they don't crush air coolers.