It's TV and if TV were realistic, I'd like to know why with all the gun play last season and people shooting at each other so much, why was everyone missing? Why, if we're being realistic, is the ammo never running out, and why was the first young man that got killed in this season's opener, holding his shotgun in such a way that he was aiming it at two of the characters instead of holding it correctly....I've given up asking such questions because TV just doesn't mirror reality, so for your question - the zombies are everywhere because it's entertaining and that's what the producers wanted to see.