Why aren't door gunner type helicopters ever used in zombie movies?

I notice in zombie movies the military always fights the zombies with foot soldiers and tanks. To me I think that is stupid. Since zombies can't fly, helicopters would be the ideal weapon during a zombie apocalypse. Not only can they mow down all those zombies with machine guns while completely out of their reach, they can also rescue survivors, deliver supplies and be able to land in any makeshift survivor camp. If I where a military commander during a zombie apocalypse, I would send helicopters, nothing but them. So why does the military always attack zombies on foot when helicopters would give a major advantage?