I think because it's building on the point you made on ARMA, the realism point. If a Zombie Epidemic actually broke out, you wouldnt be living in levels and upgrading guns like in call of doody, you would be doing this. Sneaking around, being quiet, not trusting anyone, scavenging any supplies you could find, not wasting ammo, ACTUALLY suffering from thirst and hunger. So the same reason's that you love ARMA for being a more realistic FPS, we love DayZ because it's a realistic view on what would happen. No leveling, no grinding for any kind of gear whatsoever, none of that. Just pitting yourself against the world that was destroyed, and now is out to destroy you !