The reasons for that are simple:
1. None of the Free/OSS media players support multithreading, so the 4,8,or 64 cores are wasted. This is mostly caused by ffmpeg which still, after all these years, does not do multi-threaded decoding.
2. Seeing as only one core is used, the speed becomes very important! I've ran a lot of tests with various video files, and the conclusion is that one needs at least 3Ghz for proper playback of most content. Even then there will be some movies that have trouble in certain high-movement parts, for example when there is snow, rain, or flocks of birds flying around.
As a proper test, I recommend two clips:
1. The Matrix Reloaded intro, with the "green/digital" clock -- that will run sluggish @2Ghz but should be ok @3Ghz.
2. Scenes with millions of flying birds from "Planet Earth". Those become a slideshow even at 3Ghz.
Of course, a lot of other content will play fine, and the slowdown will only be observed in high-action scenes, but that doesn't make a 2Ghz processor fit for the job, at most it's barely adequate. I would also like to see benchmarks with VDPAU for a movie that brings the processor to it's knees, otherwise these low-bitrate tests are quite useless...