This is actually kinda complicated you know.
The game is not a zombie apocalypse survival stuff, It's just sandbox survival.
Maybe in hardcore mode only, mobs could break certain blocks (sand, dirt, planks) to reach you, but adding this as an obligatory feature would be unnecessary and dangerous for the game balance. What if you don't want to have your ass bothered by IA-Griefers ?
Don't forget the gravity problem. The map would be so damn ugly after a zombe raid. Problems, problems everywhere ! One does not simply suggests something !
