Twill be a great comfort to us all to know that the issue is explained adequately for numerous undead scenarios.
Come to think of it, in the longer-developing scenarios, like THE WALKING DEAD and many of the sequel movies, you'd expect not just an explosion of the fly population, but a general increase in all manner of carrion feeders, many of which would be able to prey on moving walkers as well as on the corpses strewn all across the countryside.
Well, I've still never gotten into what really bothers me the most about THE WALKING DEAD and other mass kill scenarios: Why aren't there more flies? There would be zillions of flies. It's the zombie movie equivalent of noise in outer space.