Video encoding isn't GPU-parallelizable. It's a good fit for either CPU+SIMD or custom ASICs. It's just a kind of compression, which means it's based on unpredictable if-statements, which is just what GPUs don't do.

You can mass parallelize it by encoding different clips on different CPUs, this is more optimal because it has less communication overhead.

No idea, maybe it will change some day. But right now, if you want the best quality at the smallest filesize, nothing seems to come close to the best software encoders. Maybe x264 and x265 are just really good.

They're unusually good because 1. encoding research wasn't done on a business schedule 2. isn't done according to objective metrics (the main ones used, SSIM/PSNR, suck and you have to use human raters) 3. more varied and weirder testcases (some pirated movies, some video game screen recording, more anime).

There's a lot of other free video encoding tools, like avisynth plugins, that are just better than all professional tools. I'm not sure why this is, maybe customers aren't sophisticated enough.

What about AV1 ?