Walking Dead neatly sidestepped this with:
It's a virus, when you die, even of natural causes you become a zombie. You are never safe
Also sorts out the island/ship stuff.
Good example is in Walking Dead S1 the game.
More interesting is why go to store and only take a bit and not clear out the place instead of going back again and again.
Plus car batteries, plus not paying attention in the woods in a zombie filled world etc