well… in realtime rendering the cpu is busy enough handling assets and game code. the speed gain in rendering is minimal, anyway. also you’d have to do load balancing and resource duplication, tile management and all that stuff. the cpu would render a tile of maybe 128x128 and the gpu does the rest of 1920x1080. so they both deliver the frame in time.
in offline rendering time and sync doesn’t matter, but the cpu is still too slow. you have the same management todo. for no real gain.
blender cycles (for example) allows hybrid rendering. whether a cpu+gpu or igpu+dgpu or even cpu+igpu+dgpu. in all cases of the rendering being split across devices it’s slower, cause it’s gotta synchronize all the resources and you gotta have tile management.