Actually, video encoding for most codecs is a task which doesn't lend itself to parallelism due to temporal encoding. Because each frame depends on the last one, you really don't gain much from going to multiple cores (at least yet).
To start encoding at different times with each core you would need a really fast (read: expensive) storage (every core will need a steady stream to compress, if you have 64 cores you need a storage that can handle 64 parallel requests really fast or a lot of RAM to cache everything) and the chips have to handle really heavy I/O. The current many-core ARMs only have a small amount of dedicated RAM for each core.
Also, keyframes are not known from the start in many encodings. A keyframe is inserted when the difference between a frame and the next one is big; this can only be detected when encoding, or with a multi-pass encoding.
