I can't say for sure, but one of my guesses would be stability. By that I mean that the noise is acceptable for a single static image, because it doesn't change - but distracting in an animation if the dithering completely changes from frame to frame.

I might be a non-issue - either not noticeable or not distracting - and the real reason is that people just don't see banding as a problem or that the rendering overhead per-frame is too much.