[DungeonMaster]

I think the writers accounted for this by making people reanimate as zombies within hours — not days — after dying.

But they're still dead and stinking which would bring the flies. Zombies don't spend a lot time swatting flies so the flies would have free run on the juiciest parts of their face, ie their eyes.

[Reddy]

Yes, they are dead in the traditional sense, but in some ways they are still alive (moving, eating) so that could explain why maggots don’t destroy them, or maybe something in the virus itself could repel flies. It’s a fantasy show, explanations are endless. ;)