Because there are more steps involved. You have to ensure the mobo supports the CPU, possibly have to upgrade the BIOS, remove the CPU cooler, swap the CPU, reinstall the cooler, configure the BIOS, then you are done. With a GPU, you unplug the power connectors, swap the card, connect the power connectors, boot computer, install drivers, then done. GPU is much easier. You've obviously never done either if you are asking the question.