Agreed!
How come the TV and movie zombies never show the walking corpses covered with flies, maggots and other vermin?!? I would also expect that the crows, vultures and other carrion would make quick meals out of all of them in just a few hours.
I don't know about flies and maggots; but, this season's Walking Dead has some pretty gnarly looking zombies. I noticed it right off this season, that the zombies were looking worn out and nasty. At one point, a zombie brushes against a car and the skin on it's face just sluffs off.
I think that they realized that it has been a while since the whole apocalypse started and, as you indicate, the zombies should start looking pretty bad. So, this season the zombies are looking pretty nasty.
There’s the assumption/requirement that zombies resist decay and are poisonous to carrion eaters. Otherwise, yes, they decay or get eaten in a couple weeks.
After all, dogs would have a field day if they COULD eat zombies and not get infected.