When the zombies stop killing you as much, that means you're getting good at the game. But mark my words, somewhere, sometime down the line, you're going to get killed by a zombie. You might fuck up and break a leg, then immediately get knocked unconscious, then get munched to death. Zombies aren't that strong, but there's a lot of them, they all want to kill you, and they're standing between you and your sweet, sweet shiny stuff. They're the constant threat that keeps you on your toes in this game. Aside from psychotic bandits with bigger guns than you, they're the only lingering, always present threat that won't hesitate to fuck you over. And that's why the game is called DayZ, not PVP Survival.