Some of that is just an engineering resource problem. If you can bring yourself to watch Peter Jackson's awful King Kong remake, the giant ape looks incredibly good while creatures in the valley of the dinosaurs are dreadful...on the big screen I was able to spot that they'd been done by two different FX teams (CG/compositing geek here, so I'm abnormally fussy).

Yep. still pictures are a lot further than moving ones.

I don't think it's that stills are "further" as much as that the error doesn't accumulate.

This is actually noted in much of the 'uncanny valley' predictions, in that the positive and negative effects are less severe. A static picture can look human enough to fool you, however a video doesn't take long before something clues you in and at 24fps you're likely to clue in fast.

However zombies don't look as horrifying in still pictures as they do in animation. Catching the bottom of the uncanny valley can have its benefit for directors as well.