Common video codecs are often hardware accelerated. This should be on the CPU side quite often, as there are a lot of systems without dedicated GPUs that still play video, like Notebooks and smart phones. So in the end it's less about being parallelizable, but if it beats dedicated hardware, to which the answer should almost always be no.

P.S.: In video decoding speed is only relevant up to a certain point. That being: "Can I decode the next frame(s) in time to show it/them without stuttering". Once that has been achieved other factors such as power drainage become more important.

I think it's mostly because most cpus that can run a gpu already have parts dedicated as h264 encoder, way more efficient energy wise and speed wise.

A GPU's job is to take inputs at some resolution, transform it, and then output it at that resolution. H.264/H.265 (and really, any playback format) needs a fundamentally different workflow: it needs to take as many frames as your framerate is set to, store the first frame as a full frame, and then store N-1 diffs, only describing which pixels changed between each successive frame. Something GPUs are terrible at. You could certainly use the GPU to calculate the full frame diff, but then you still need to send it back to the CPU or dedicated encoding hardware that turns that into an actual concise diff description. At that point, you might as well make the CPU or hardware encoder do the whole job, you're just not saving any appreciable time by sending the data over to the GPU first, just to get it back in a way where you're still going over every pixel afterwards.