Most standard video decoding code uses CPU feature detection and hand-optimized ASM for important bits, so x86-64 doesn't really add anything worthwhile for most video players and encoders - most of the magic is done in SSE/AVX codepaths anyway.
And x86-64 turns those 8 SSE registers into 16! Those handwritten assembly paths are much easier when you don't have to write stack spills.
Hmm, you have a point there - x64 does make writing ASM significantly easier.
Hardware decoding is much more important for power savings than anything else.