It's not really the point of a zombie movie, or any movie or story containing magical, free-energy beings/things/spells/doodads for that matter. It's just an accidental consequence of the magic/fantasy.

For zombies the point (and the plot) should always revolve around the existential dread of relentless, never-ending hordes of beings who want to eat your brains.

It's like the old story about the very best thing superman could do is simply turn a crank powering the world's energy. All that flying around saving people is actually a massive, inefficient, effectively people-killing, waste of time.

Try writing many series of comics about that though!