I like zombie movies as much as the next guy, but if they made sense, they wouldn't be a zombie movie.
Characters in a zombie movie never realize time is on their side. Just barricade yourself in a grocery store, and eat canned food for a couple of months, while the undead decompose outside. Then, after a couple of months, go outside. Presto, problem gone. Of course, that would make one of the worst movies ever, so they have to make the characters do frustratingly irritating things like leave their safe place in order to go drive to the next city, or check on Uncle Ned, or something where they call attention to themselves and then get killed.
Like the movies about aliens invading earth for the water.... do they not know how much water ice there is in the asteroids, comets and other planets? Why fight a war for it when we couldn’t possibly put up a defense?