You can find GPUs made by nvidia and ati as reference cards, or third parties like galaxy and evga, but CPUs are only made by Intel and and. you used to be able to buy clone cpus from Motorola, cyrix and st back in the early 90's. What happened?
This might be bcoz everyone who uses a pc uses a processor(cpu) . So intel and amd sell to larger audiences and hav larger profits. and so they hav more profits then they wud hav had if they had partnerships with third parties providing their chips and designs to them. However not every1 who uses a pc uses a dedicated gpu. I am yet to see an office which does not deal into graphics designing/gaming which provides dedicated gpus to each and every pc. And offices are the main customers of the pc. SO nvidia and amd make chips for gamers/ graphics designers/3d modelers. If they had mass manufacturing units for these, they wud hav gone into losses, bcoz of supply being more than demand. So they sell their designs to smaller scale manufactureres who manufacture and cater to a smaller audience(gamers). In this way they make money. in the end everything said and done, all their chips arre for profit. and selling them to third parties is more profitable. After all the whole worldruns on money. this is my hypothesis