But that doesn't happen - when you're encoding, the DCT isn't actually run on the image but on the output of previous compression steps (prediction) which are based on the last encoded block. So there's a dependency on every pixel of the image to the upper left of you.
And when you're decoding, it just never ends up worth it to read the whole bitstream so you have a whole frame of motion vectors to do it at once. The whole data locality thing.
But that doesn't happen - when you're encoding, the DCT isn't actually run on the image but on the output of previous compression steps (prediction) which are based on the last encoded block. So there's a dependency on every pixel of the image to the upper left of you.
And when you're decoding, it just never ends up worth it to read the whole bitstream so you have a whole frame of motion vectors to do it at once. The whole data locality thing.