I don't think you understand how this works.
Now, when we propose a fantasy or 'science fiction' scenario, we add in something unrealistic. Space aliens. Zombies. Vampires. Dragons. Whatever. However, everything else should remain realistic. In this genre, we have zombies. However, it ruins the immersion to then add, "also I'm a hobbit, and can magically dig underground with ease, and yes there's a zombie apocalypse above ground, but below ground I can live quite safely without any concerns whatsoever about air quality or flooding or explosive gases.".
The addition of one unrealistic element does not then warrant the inclusion of more such elements. Indeed, adding additional unrealism will break immersion.
The 'rules' of the zombie genre are well defined. In this case, instead of the Romero style, the developers have adopted the DOOM variant (also used by Left for Dead). I disagree with their choice, but whatever. Unfortunately, this genre does not permit magical underground digging. I think the game you are looking for is called Minecraft, which makes no effort to maintain realism.
I will agree that digging zombies are a bit much, but they are a necessary means of forcing underground dwellers to actually play the game. An even better solution would be to make stone much more difficult to dig through, much heavier, and add in pockets of explosive methane and a much higher increase of a cave-in. Also, when it rains, people in tunnels should drown like rats.