Why has no one done a zombie mod?

[Linelor]

This game would be so perfect for a zombie mod, I'm really confused as to why no one has done it. I don't have any coding knowledge or I would attempt it myself, but I figure you could probably make one relatively quickly using the burned skin for models since there's already a melee attack in the game.

I searched and didn't find anything on it so I really can't figure out any problems with it.